Direct Synthesis of Nanosized NiCo2O4 Spinel ... - ACS Publications

Jun 2, 2007 - Department of Chemistry, University College London, Christopher Ingold Laboratories, 20 Gordon Street, London WC1H 0AJ, U.K.; The ...
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Direct Synthesis of Nanosized NiCo2O4 Spinel and Related Compounds via Continuous Hydrothermal Synthesis Methods Paul Boldrin,†,‡ Andrew K. Hebb,‡ Aqif A. Chaudhry,†,‡ Lucy Otley,§ Benedicte Thiebaut,§ Peter Bishop,§ and Jawwad A. Darr*,† Department of Chemistry, UniVersity College London, Christopher Ingold Laboratories, 20 Gordon Street, London WC1H 0AJ, U.K.; The Department of Materials, Queen Mary UniVersity of London, Mile End, London E1 4NS, U.K.; and Johnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, U.K.

A series of crystalline homometallic and heterometallic cobalt and nickel hydroxides and oxides were prepared using a continuous hydrothermal flow synthesis system. In all syntheses, the relevant metal salt solutions were pumped under high pressure to meet pH or other chemical modifiers (H2O2 or PVP) before the mixture was brought into contact with a feed of superheated (or supercritical) water, whereupon precipitation and particle growth occurred. The resulting nanoparticle (typically less than 100 nm in diameter) suspensions were collected from the outlet of the back-pressure regulator of the hydrothermal system. The collected suspensions were centrifuged, and the washed solids were freeze-dried prior to analyses. The nanopowders were characterized by a number of analytical methods including X-ray powder diffraction, Brunauer-EmmettTeller (BET) surface area measurements, and simultaneous thermogravimetric analysis/differential scanning calorimetry. 1. Introduction The production of homogeneous nanosized binary or higher ceramics is of interest in applications such as catalysis,1 battery materials,2 electronic storage media,3 and solid-state sensors,4 as the reduction in size of the materials can improve certain properties because of quantum-confinement effects or greatly increased surface area.5-7 For example, nanoparticles of materials such as NiO can exhibit interesting magnetic properties (including high coercivity, large magnetic moments, and loop shifts)8 or as a sensor for CO, while Co3O4 has been used as a sensor for H2 and CO, respectively.9 Nickel and cobalt based nanomaterials are of interest in catalysis as inexpensive replacements for noble metal catalysts. For example, Co3O4 can catalyze the oxidation of CO,10 while NiO can be used as a cocatalyst for the photocatalytic splitting of water.11 In battery materials, Ni(OH)2 (both pure12 and doped with Co13) has been used as an anode material. CoO(OH) has also been reported as an anode in nickel batteries,14 while Co3O4 and CoO can also be used as electrode materials in lithium-ion batteries.15 In many of these applications, the particle properties, such as size, surface area, and crystallinity, have a profound effect on the performance of the material. A number of routes have been employed for the manufacture of cobalt or nickel oxide or hydroxide nanoparticles.16-23 For example, Co3O4 has been prepared by mechanochemical synthesis methods16 or by the thermal decomposition of Co(NO3)2.17 Co3O4 and CoO can be produced by chemical vapor deposition (CVD)18 and plasma deposition methods.19 Amorphous Ni and Co hydroxide solid solutions are known to convert to a mixture of hetero- and homometallic oxides upon heat treatment.20 Furthermore, Ni(OH)2 has also been prepared in a hydrothermal batch reactor21 and by thermal decomposition22 of either the nitrate or chloride salts with urea (particle size * To whom correspondence may be addressed. Fax: +44 8701303766. Tel.: +44 2078825191. E-mail: [email protected]. † University College London. ‡ Queen Mary University of London. § Johnson Matthey Technology Centre.

range ) 0.9-3.3 µm). CoO(OH) can be produced by chemical or electrochemical oxidation of Co(OH)2.23 Many of the aforementioned methods of synthesis have drawbacks in that they may be energy-consuming, lengthy, and generally not considered green (in terms of atom efficiency) and involve multiple steps. Therefore, more environmentally friendly, faster, and more energy-efficient synthesis methods for nanoparticle production are currently of interest. In the manufacture of crystalline inorganic nanomaterials, hydrothermal methods (using supercritical or subcritical water) often require relatively lower synthesis temperatures compared to more conventional methods.24,25 Superheated water is of interest for a number of other useful applications (e.g., destruction of toxins) because it retains many useful properties at nearcritical or above its critical point (critical temperature and pressure are 374 °C and 22.0 MPa, respectively).26 These properties, which have been reviewed recently,27 include rapid reaction kinetics, tunable dielectric constant, ability to solubilize some inorganics, and high solubility of dissolved gases. The vast majority of inorganic hydrothermal syntheses tend to be conducted in batch reactions, which can be time-consuming and give little or no control over particle properties or phase composition.28 In 1992, Arai’s group in Japan published a paper detailing a new continuous hydrothermal flow synthesis (CHFS) method for making nanomaterials.29 It involved mixing a stream of superheated water with a stream of metal ions in solution, with the rapid change in the environment of the metal ions causing them to precipitate as nanoparticles. This method was soon shown to be applicable to the production of a variety of metal oxides and hydroxides such as Fe2O3, Co3O4, NiO, ZrO2, and TiO2.30 This early work produced particles that were often large, with wide particle-size distributions, and, in selected cases, were produced in very low yields. Because of these limitations, a third pump was added to some of the flow system designs to premix with the metal salt solution (before the mixing of superheated water), initially to produce an alkaline environment31 but later to produce materials that were not previously possible because of mutual insolubility of precursors when

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Figure 1. Schematic representation of the three-pump (P1, P2, and P3) hydrothermal flow system used for the preparation of nanoparticles. Key: P ) pump, C ) cooling, F ) filter, B ) back-pressure regulator, R ) countercurrent reactor (this can be also heated externally with an additional band heater), and T ) “Tee” piece mixer.

mixed.32,33 Also, the order of mixing of the three feeds (i.e., hydroxide ions meeting metal ion or superheated water first) was found to have a significant effect on the particle size, dispersity, and shape.34,35 Several papers using CHFS have made efforts to control the particle size. Arai’s group found that particle size decreased with increasing supersaturation during the synthesis of a variety of simple metal oxides.36 Meanwhile, Lester’s group in the U.K. recently modeled the mixing point of the reactor in order to improve the uniformity of particles and reduce the problem of blockages,37 and later they published a design for a vertical coaxial countercurrent mixing design that takes advantage of the predominance of buoyancy over viscosity in the continuous hydrothermal system.38 They showed that this mixing-point design gave good consistency for surface-area values obtained in the particles produced between “batches”. More recently, many different materials have been produced using CHFS technology, including simple metal oxides,30,34,35,39,40 mixed metal oxides,25,31,33,41-44 solid solutions,45,46 and metals.47,48 Almost all of the papers published so far in this area have concentrated on the production of a single target material per metal ion (or combination of metal ions), with temperature and pressure (and occasionally pH) being varied within a range in order to vary the particle size (see, for good examples, papers by Arai’s group on AlO(OH)49 and YAG50). In addition, there have been few reported uses of H2O2 in continuous hydrothermal synthesis,41,51-53 despite its extensive use in continuous supercritical water oxidation, and these have only looked at its oxidizing effect. Herein, we will demonstrate that different phase pure nanomaterials with modulated sizes and shapes can be produced from the same basic starting metal ion(s) by variation of temperature, pH, and the use of H2O2 and other additives. Finally, a number of materials, particularly the NiCo2O4 spinel, were directly synthesized, which have not previously been produced via the CHFS technique. 2. Experimental Section All experiments were conducted using a continuous hydrothermal flow synthesis system, the details of which are described elsewhere.32 The reactor, tubing, and components were all made of 316 stainless steel (Swagelok). The apparatus (Figure 1) consists of a metal salt solution pump, P2 (Gilson model 305/ 5SC pump head); a base solution pump, P3 (Gilson model 305/ 10WTi pump head); and a H2O pump, P1 (Gilson model 305/ 10SC pump head). H2O was pumped through an electrical

preheating coil (2.5 kW) and heated to the appropriate temperature (given in Table 1). Flow rates of 5, 5, and 10 mL/min were used for the salt solution, base solution, and water streams, respectively. The system pressure was maintained at 24.1 MPa by a Tescom back-pressure regulator (BPR), model 26-176224-194. In our hydrothermal process, the metal salt solution (which could contain additives) and the base solution, respectively, were first pumped and brought to mix in a stainless steel Swagelok 1/ in. “Tee” piece approximately 5 cm below the point of 8 contact with the superheated water. The superheated water feed was then brought into contact with these premixed metal salts and additives (see Table 1), which were initially at room temperature, using a vertical coaxial countercurrent mixing arrangement as described elsewhere.32,38 The mixing point (Figure 2) consists of a 1/8 in. o.d. 316SS tube that extends downward through a 3/8 in. 316SS Swagelok cross-piece and ca. 5 cm into a 3/8 in. o.d., 0.77 cm i.d. 316SS tube. The superheated water flows downward into the mixing point through the 1/8 in. tube, and the metal salt solution flows upward through the 3/8 in. tube. The point at which the 1/8 in. tube ends is where the initial mixing occurs. Of the horizontal exits to the cross-piece, one is a 3/8 in. diameter 316SS rod, which has a 1/16 in. hole drilled to within 0.1 cm of its end, in which is a k-type thermocouple. This thermocouple is 7.0 cm above the initial mixing point, and the temperatures measured here are quoted as the mixing-point temperatures in Table 1. The final exit is 3/8 in. o.d. 316SS tube of ca. 10 cm in length that has a 90° bend and leads downward into the cooler. In selected experiments, an additional electrical heater was used near the mixing point (on the countercurrent mixer). The heater is a 200 W Watlow band heater, of 2.5 cm diameter. It is in thermal contact with the 3/8 in. tubing in which the initial mixing takes place through a 2.5 cm o.d., 3/8 in. i.d. duralumin tube that is mounted with the 3/8 in. tube passing through its center and positioned such that the bottom of the heater is at the same level as the end of the 1/8 in. tube. In the duralumin tube is mounted a k-type thermocouple, which is used for control of the heater. The entire mixing-point arrangement is shown in Figure 2. At the end of the mixing point, the mixture was cooled using an external water cooled jacket, C. The suspensions were then passed through a 7 µm filter, F, to remove any larger aggregates that might affect smooth operation, before exiting the back-pressure regulator, where they were collected. Solids were recovered by centrifuging the suspension (5 min at 4500 rpm), decanting off the liquid phase, and washing the solids thoroughly with clean water. This procedure was repeated once more, the liquid phase was decanted, and the concentrated slurry was freeze-dried. Materials. Cobalt(II) nitrate hexahydrate (Aldrich, U.K., 98%), nickel(II) nitrate hexahydrate (Aldrich, U.K., 98%), nickel(II) acetate tetrahydrate (Riedel-de Hae¨n, Germany, min 98%), potassium hydroxide (Acros Organics, U.K., purum grade), hydrogen peroxide (Aldrich, U.K., 35 wt % solution in water), and polyvinylpyrrolidone, PVP (Fluka, U.K., K15, Mr ≈ 10 000) were used as obtained, without any special procedures to exclude air/moisture. All experiments were conducted using 18 MΩ deionized water, which was not degassed. Characterization. Freeze-drying was performed using a Vertis Advantage freeze-dryer at ca. 10-6 MPa for a period of 12 h. X-ray powder diffraction studies were conducted on a Siemens D5000 X-ray diffractometer using Cu KR radiation (λ ) 0.154 18 nm). Data were collected over the 2θ, range 15100°. The particle size and morphology of the prepared powders

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Table 1. Details of Experimental Conditions and Products Obtained for the Hydrothermal Flow Reactionsa expt

metal feed concentration

1 2 3 4 5 6 7 8 9 10 11 11h 12 13 14 15 16 17 18 19 20 21

0.1 M Co 0.1 M Co 0.1 M Co 0.1 M Co 0.1 M Co 0.1 M Co 0.1 M Co 0.1 M Co 0.1 M Ni1 0.1 M Ni1 0.1 M Ni1 0.1 M Ni1 0.1 M Ni2 0.1 M Ni2 0.1 M Ni2 0.1 M Ni2 0.1 M Ni2 0.095 M Ni2+0.005 M Co (19:1 ratio) 0.08 M Ni2+0.02 M Co (4:1 ratio) 0.05 M Ni2+0.05 M Co (1:1 ratio) 0.033 M Ni2+0.067 M Co (1:2 ratio) 0.033 M Ni1+0.067 M Co (1:2 ratio)

KOH conc/M 0.2 M 0.2 M 1.0 M 0.2 M 0.2 M 1.0 M 1.0 M 0.2 M 0.2 M 0.2 M 0.2 M 0.2 M 0.2 M 0.2 M 0.2 M 0.2 M 0.2 M 0.2 M 0.2 M 0.2 M

additives

0.8 M£ 0.1 M$ 0.8 M£ 0.8 M£ 0.8 M£

0.8 M£

temperature of water/°C

mixing point temperature/°C

product(s) from XRD data

450 450 450 450 350 350 350 350 450 450 450 450, + 200 250 300 350 400 350 350 350 350 450, +

∼290 ∼290 ∼290 ∼290 ∼190 ∼190 ∼190 ∼190 ∼290 ∼290 ∼290 ∼310 ∼90 ∼110 ∼135 ∼160 ∼255 ∼160 ∼160 ∼160 ∼160 ∼310

Co3O4 Co3O4 Co3O4 CoO Co(OH)2 Co(OH)2 CoO(OH) Co3O4 no reaction β-Ni(OH)2 and NiO β-Ni(OH)2 and NiO NiO β-Ni(OH)2 β-Ni(OH)2 β-Ni(OH)2 β-Ni(OH)2 β-Ni(OH)2 Ni-Co-OH SS Ni-Co-OH SS Ni-Co-OH SS Ni-Co-OH SS NiCo2O4

particle size/nm 103* 42* 16* 47 22 13 9 14 9 (44) 10 (11) 3* 4 4 4 6 13 4 5 5 11 6*

SA/m2g-1 8 n/a n/a 33 64 77 56 182

167 90 96 68 94 76 58 73 163

a Key: Ni1 ) Ni(NO ) , Ni2 ) Ni(Ac) , £ ) H O , and $ ) PVP. Experiments marked + used a Watlow band heater after the mixing point (set at 450 °C 3 2 2 2 2 in all cases). Particle sizes marked * are averages from TEM; all others are from XRD peak halfwidths. For experiments 10 and 11, the particle size in brackets is for the minor nickel oxide phase only. Surface areas are given only for experiments with enough product to conduct reliable BET measurements; SS ) solid solution and SA ) surface area.

Figure 2. Figure showing the dimensions of the stainless steel pipes and band heater in the countercurrent mixer of the continuous hydrothermal flow synthesis system.

were observed using a JEOL 2010 transmission electron microscope (200 kV accelerating voltage). Simultaneous differential scanning calorimetry (DSC) and thermogravimetric analyses (TGA) were carried out using a Polymer Labs STA 1500 at a heating rate of 10 °C‚min-1 in dry N2. Selected samples were heat-treated in air using a Carbolite furnace (model RHF1600) at 400 °C for 1 h. Magnetic susceptibility measurements were carried out using a Sherwood Scientific magnetic susceptibility balance Mk1. 3. Results and Discussion Nanopowders were synthesized in a continuous hydrothermal synthesis system built in our laboratory, the basic design of which is described elsewhere.32 Table 1 shows the experimental parameters used; concentrations of solutions, superheated water temperature, mixing-point temperature, identity of the products, and particle sizes are given. The metal salts used herein were cobalt(II) nitrate hexahydrate, nickel(II) nitrate hexahydrate, and nickel(II) acetate tetrahydrate, respectively. Typically, the total concentration of cobalt or nickel ions (or any mixture of them) was set at 0.1 M. The concentration of the base (KOH) in an

aqueous solution that was pumped under pressure to premix with the metal salt(s) was either 0.0, 0.2, or 1.0 M (see Table 1). In selected cases, hydrogen peroxide of up to 0.8 M was used in addition to base (in the same solution feed), and in one case, polyvinylpyrrolidone (PVP) was added to the base. All reactions were conducted using a superheated water feed up to a temperature of 450 °C (mixing-point temperatures are also quoted, but it should be remembered that the thermocouple measuring these is ca. 7 cm above the initial meeting point and that the temperature can vary by 5-10 °C above or below the quoted temperature during a run). After the hydrothermal reaction in the countercurrent mixing zone, particles were cooled and collected from the exit of the back-pressure regulator as a fine suspension. After cleanup, the respective slurries were freeze-dried overnight (12 h) at ca. 10-6 MPa. Phase identities and purities of the dried materials were elucidated using X-ray powder diffraction (XRD), and particle sizes were estimated from transmission electron microscopy (TEM) images and from calculations based on the XRD peak halfwidths (applying the Scherrer equation).54 The results are summarized in Table 1. From Table 1, it is evident that all but one of the cobalt(II) nitrate experiments conducted using sc-H2O at 450 °C (including

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Figure 3. XRD plot of Co3O4 prepared using a superheated water inlet temperature of 450 °C and 24.1 MPa pressure in the hydrothermal flow reactor (experiment 1).

those without peroxide) produced phase pure spinel Co3O4 (Figure 3). The exception to this was formation of phase pure CoO at this temperature when excess base (1.0 M) was used. In Co3O4 spinel, two-thirds of the cobalt ions are Co(III) and one-third are Co(II). In comparison, Caban˜as and Poliakoff41 previously reported that sc-H2O (in the range 200-400 °C), in a flow system, reacted with Co(Ac)2 to give CoO as the major product. This suggests that a stronger oxidizing environment is present in our experiments. This is presumably due to our use of nitrates rather than acetates; in supercritical water, the former decompose to form various oxidizing species including hydroxyl radicals, O2 and NO2.55 When the superheated water feed temperature was reduced to 350 °C, phase pure Co(OH)2 was produced with 0.2 M base. However, when a higher concentration of base (1.0 M) was used at this temperature, phase pure CoO(OH) was formed. Thus, at higher base concentrations, the OH- ions can strip a proton from the Co(OH)2 to form CoO(OH) in which the Co2+ is oxidized to Co3+. Co(OH)2 and CoO(OH) have not been produced using continuous hydrothermal flow synthesis before. Interestingly, the addition of peroxide under these conditions produces Co3O4, in which, on average, the Co is actually more reduced than in CoO(OH), indicating that the peroxide is not simply an oxidant, as would be expected. These results show that it is possible to obtain a number of phase pure cobalt compounds starting from cobalt nitrate, by simply varying the temperature and pH and by adding H2O2. The data also suggest that it is possible to widen the envelope in which certain materials can be formed by changing all three parameters together. For example, in our reactor, without base or peroxide, Co3O4 can only be produced using a sc-water feed temperature of 450 °C or above. In contrast, by adding, and varying, the amounts of base and peroxide, it is possible to produce Co3O4 at 350 °C (sc-water feed temperature). This allows greater manipulation of particle size and/or morphology, by increasing the range within which the different variables can be changed while maintaining phase pure products. When polyvinylpyrrolidone (PVP) was added to the metal salt feed at 350 °C (0.2 M base) and no peroxide, phase pure Co(OH)2 was obtained as expected (experiment 6). Furthermore, a significant reduction in particle size was observed with PVP addition (this was calculated from XRD data to reduce from ca. 22 to 13 nm for experiments 5 and 6, respectively). Attempts to repeat experiment 6 at 450 °C resulted in significant PVP decomposition; thus, the results for this experiment are not discussed further herein. Figure 4 shows the XRD patterns for the experiments with and without PVP (experiments 6 and 5, respectively).

Figure 4. XRD plot of Co(OH)2 prepared using a superheated water inlet temperature of 350 °C and 24.1 MPa pressure in the hydrothermal flow reactor with PVP (top) and without PVP (bottom) for experiments 6 and 5, respectively.

Figure 5. TEM images of Co3O4 particles produced (a) without the addition of base or peroxide (experiment 1, bar ) 200 nm); (b) with 0.2 M base (experiment 2, bar ) 20 nm); and (c) with 0.2 M base and 0.8 M peroxide (experiment 3, bar ) 20 nm).

Parts a, b, and c of Figure 5 show TEM images of Co3O4 particles from experiments 1, 2, and 3, respectively (water feed at 450 °C). Figure 5a shows a TEM image of Co3O4 produced with no base or peroxide (experiment 1), which shows particles with a mean particle size of ca. 100 ( 59 nm (207 particles sampled). The particles have clearly defined edges and a wide particle-size distribution in the range ca. 15-357 nm. Figure 5b shows a TEM image of Co3O4 (made using 0.2 M KOH), showing more regularly shaped particles with a mean diameter of ca. 42 ( 26 nm (201 particles sampled; bar in Figure 5b ) 20 nm). Particles were observed in the range ca. 11-208 nm. A TEM image of the product from experiment 3 (0.2 M base and 0.8 M peroxide) is shown in Figure 5c and reveals the particles to be rectangles or rhomboids with a small mean particle size of ca. 16 ( 6 nm (sample of 100 particles) and a size range ca. 6-39 nm. These results clearly show the effect of chemical additives on the particle size of products. A reduction in particle size can be explained through the kinetics of particle formation. For the formation of small particles, it is necessary to increase the rate of nucleation of new particles and to reduce growth.56 As such reactions in superheated water proceed via a two-step hydrolysis-dehydration mechanism (as described elsewhere),30 the initial formation of hydroxide is increased by the presence of base and/or peroxide. Therefore,

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Figure 6. XRD plots of Ni(OH)2 and NiO mixtures prepared using a superheated water inlet temperature of 450 °C and 24.1 MPa pressure. The reactions used Ni(NO3)2 as a starting material and 0.2 M KOH (top, experiment 10), 0.2 M KOH and 0.8 M H2O2 (middle, experiment 11), and 0.2 M KOH and 0.8 M H2O2 (bottom, experiment 11h) as modifiers. In addition, for experiment 11h, an external band heater set at 450 °C was used on the countercurrent mixing point.

the growth of nanoparticles will be limited by the fact that there will be more centers of growth. These results show that H2O2 can be an effective tool for reducing particle size and/or obtaining different particle morphologies. The results from the nickel nitrate reactions are in contrast to those for the corresponding cobalt salt, as there is no visible reaction for the former salt when base was absent (experiment 9). Similar results to this have been observed previously.34,41 In experiments 10 and 11 (sc-water feed at 450 °C and with added base at 0.2 M) with and without peroxide (0.8 M), respectively, the major product of β-Ni(OH)2 was identified from XRD data. Additionally, a minor phase of NiO was observed. Yet again, this was in contrast to the analogous reaction for the cobalt salt, which gave Co3O4. Thus, the nickel remained in the +2 state as expected. XRD data of the products from the nickel nitrate reactions are shown in Figure 6. The addition of peroxide (with base) to the reagents gives a greater proportion of NiO (experiment 11), suggesting that dehydration of the hydroxide to oxide was accelerated. This was accompanied by a change in color from green/olive to black. Calculation of the crystallite size of the hydroxide phase shows virtually no size change with peroxide on the reaction, while the size of the nickel oxide phase crystallites decreased significantly from 44 to 11 nm. With the addition of an external band heater set at 450 °C at the countercurrent mixer to maintain a high reaction temperature during and after initial mixing, phase pure NiO was exclusively formed (experiment 11h), i.e., the reaction went to completion (by not heating the mixing zone, of course cooling due to mixing of hot and cold feeds simply occurs and the conditions are not right for exclusive formation of NiO). TEM images of this sample (Figure 7) show extremely small particles of 3 ( 1 nm (sample of 57 particles; range 2-8 nm). XRD data analysis suggests a crystallite size of ca. 5 nm, while Brunauer-Emmett-Teller (BET) surface area after freeze-drying was found to be 167 m2 g-1. Previous reports of batch hydrothermal syntheses for nickel hydroxide (at 200 °C) yielded R-Ni(OH)2,57 which possesses a layered structure. In the R-phase, chemical species can become intercalated (between layers), leading to an increased interlayer spacing.58 Furthermore, the amount of intercalated water and acetate was reported to decrease with increasing reaction temperature.57 The more stable β-Ni(OH)2 also has a layered structure (similar to brucite), but with no intercalated species. Accordingly, previous researchers reported that, upon increasing

Figure 7. TEM image of NiO prepared using superheated water inlet temperature of 450 °C and pressure of 24.1 MPa. An additional band heater set at 450 °C was also used at the mixing point (experiment 11h, bar ) 20 nm).

Figure 8. XRD patterns of Ni(OH)2 prepared in the continuous hydrothermal synthesis system using preheated water inlet temperatures in the range 200-400 °C. Table 2. Thermogravimetric Weight Loss Percentages Observed for Each Stage for Samples 11-16 experiment

stage 1/%

stage 2/%

stage 3/%

stage 4/%

12 13 14

7.8 6.3 10.5

17.8 15.4 18.2

9.2 6.7 7.3

3.8 2.9 2.7

the temperature, the structure shifted from pure R-Ni(OH)2, through a disordered mixed phase, to pure β-Ni(OH)2 that has little or no intercalated species. This was previously confirmed by simultaneous thermogravimetric analyses.58 Under the conditions used in experiments 12-16, phase pure β-Ni(OH)2, rather than any R-Ni(OH)2, was identified from XRD data (Figure 8). The continuous hydrothermal synthesis of phase pure β-Ni(OH)2 has not previously been reported. Simultaneous thermogravimetric analyses, STA (simultaneous TGA and DSC), data for selected nickel hydroxide nanopowders were conducted to assess if intercalated species might be present. The STA plots are detailed in Table 2. DSC data for the samples from experiments 12-16 (made using a sc-water feed in the temperature range 200-400 °C) all revealed peaks in the ranges ca. 30-130 °C (stage 1), 220-280 °C (stage 2), 320-370 °C (stage 3), and 390-410 °C (stage 4), which can be assigned to loss of water, dehydration of the hydroxide to form the oxide, loss of the residual acetate groups, and, finally, decomposition of the mixed oxide to form nickel metal, respectively. The largest weight losses were attributed to the dehydration process (stage 2). In addition, the overall weight losses observed from the TGA plots up to 450 °C are similar in all the samples (weight losses in the range 28-34%). However, TGA/DSC data for sample 16 showed marked differences compared to the previous samples (Figure 9 top trace); less water and acetate were lost from this sample in stages 1 and 3, respectively. Because samples 12-16 are all β-Ni(OH)2, it is presumed that the acetate

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Figure 11. Calculated lattice parameters against molar Co content (%) for pure Ni(OH)2, Ni-Co hydroxide solid solutions, and pure Co(OH)2 (assuming hexagonal unit cell). The hollow symbols were calculated from the corresponding JCPDS line pattern positions of Ni(OH)2 and Co(OH)2 (JCPDS patterns 14-0117 and 03-0913, respectively).

Figure 9. (a) Thermogravimetric analysis traces for Ni(OH)2 produced at 400, 300, and 200 °C (from top to bottom), for experiments 16, 14, and 12, respectively, and (b) the corresponding differential scanning calorimetry traces of Ni(OH)2 produced using superheated water inlet temperatures of 400, 300, and 200 °C (from top to bottom).

Figure 12. DSC traces in the range 30-450 °C of pure Ni(OH)2, Ni-Co hydroxide solid solutions, and pure Co(OH)2 that were prepared using a superheated water inlet temperature of 350 °C for experiments 15, 17, 18, 19, 20, and 5 (from bottom to top, respectively; percentage shown is mol % Co).

Figure 10. XRD patterns of pure β-Ni(OH)2, Co-Ni hydroxide solid solutions and pure Co(OH)2 prepared using superheated water inlet temperature of 350 °C in the hydrothermal flow system (experiments 15, 17, 18, 19, 20, and 5, respectively, from top to bottom; percentage shown is mol % Co).

resides on the surface of particles rather than being intercalated. The lower amount of acetate present in sample 16 is attributed to greater oxidizing conditions experienced during the growth of these particles. The hydrothermal flow system (using preheated water at 350 °C) was also used to prepare a series of Ni-Co hydroxide solid solutions in 19:1, 4:1, 1:1, and 1:2 molar ratios, respectively (experiments 17-20). Figure 10 shows the XRD plots of the nanopowder products from these experiments, together with those of experiments 15 and 5 for comparison (pure Ni(OH)2 and pure Co(OH)2, respectively). It can be seen that Co(OH)2 and Ni(OH)2 have a similar trigonal structure, isomorphous with brucite,59 with the former having slightly smaller lattice parameters due to the smaller Co2+ ionic radius. The XRD patterns of the mixed cobalt-nickel hydroxides are, as expected, shifted relative to the two corresponding hydroxides depending on the Ni/Co ratio. Figure 11 shows the variation in the lattice parameters a and c with Ni/Co molar ratio (calculated using XRD peak positions and UnitCell software package with data fitted to a hexagonal unit cell).54 The results suggest that the a lattice parameter increases with increasing amounts of cobalt. The XRD patterns for Ni(OH)2 and CO(OH)2 (matched to JCPDS patterns 14-0117 and 03-0913, respectively) suggest that

lattice parameter a should be larger in the latter. Thus, larger values for a would be expected with higher cobalt contents in the solid solutions. Calculated lattice parameter c increases much more gradually in the solid solutions, except for a sudden decrease for the solid solution containing 5% cobalt. Decreases in the interlayer spacing have previously been reported at low levels (0-25%) of cobalt incorporation.60,61 With changing Ni/Co composition in the solid solutions, subtle changes were observed in the DSC plots (Figure 12); the endotherm at ca. 250 °C became similar to that observed for pure cobalt hydroxide with an increasing amount of cobalt in the solid solution. An endotherm centered at ca. 400 °C due to decomposition of NiO to Ni metal also became less intense with increasing amounts of cobalt. For the DSC plot of pure cobalt hydroxide, no endotherm near 400 °C was observed, as Co3O4 is stable until above 450 °C. STA data for sample 20 (1:2 Ni/Co ratio hydroxide solid solution) were slightly different from the other samples. The dehydration endotherm in the DSC plot occurred at a significantly lower temperature (235 °C), and there was no endotherm for the formation of Ni metal at around 400 °C (experiment 20). This was significant, and it strongly suggested that the corresponding heterometallic hydroxide solid solution had formed pure NiCo2O4 spinel at ca. 300 °C or so. To confirm our hypothesis, this powder was heat-treated in air in a furnace at 400 °C for 1 h. XRD data analysis of the heattreated sample revealed that a phase pure NiCo2O4 nanomaterial was indeed formed. The broadness of the XRD peaks suggested a crystallite size of ca.11 nm, which is similar to the initial solid solution. BET surface area measurements gave a surface area

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Figure 13. XRD plot of phase-pure NiCo2O4 spinel produced directly in the hydrothermal reactor (experiment 21) using a supercritical water inlet temperature of 450 °C and with a band heater at the mixing zone set to 450 °C.

Figure 15. Calculated number of unpaired electrons per metal atom against molar Co content (%) for Ni(OH)2, Ni-Co hydroxide solid solutions, and Co(OH)2. The heavy line shows the expected theoretical trend.

clear why there is a sudden decrease for Ni0.33Co0.67(OH)2, but it can be seen from the XRD pattern of this sample in Figure 9 that it possesses very low crystallinity, and the two facts may be related. The effective number of unpaired electrons for this sample appears to be close to two, which would be expected if all the Co was trivalent; however, the structure is still definitely brucite-like. 4. Conclusions

Figure 14. TEM images of NiCo2O4 from experiment 21. The main image is a dark field image of the sample highlighting the particles in white (bar ) 20 nm). The two smaller images on the right are an electron diffraction pattern (top) and a bright field image (bottom, bar ) 20 nm).

of ca. 52 m2/g, which is comparable to the corresponding asproduced hydroxide powder (73 m2/g). The relatively low temperature of formation for NiCo2O4 spinel formation in air inspired us to attempt to make this material directly in the hydrothermal system (experiment 21), using more severe conditions (inlet water was at 450 °C) and the same ratio of Ni/Co. As in experiment 11h, for experiment 21, we additionally used peroxide and a band heater set to 450 °C on the countercurrent mixer. XRD analysis of the dried black powder from experiment 21 revealed phase pure NiCo2O4 was indeed obtained (Figure 13). This powder has a surface area of 163 m2 g-1 (BET), which is more than three times the value for surface area of the heat-treated powder from experiment 20. The TEM images (Figure 14), indicate that the particles are mainly cubes in shape. Analysis of the images indicates a particle size of 6 ( 2 nm (from 79 particles; size range 3-15 nm), which is in relatively good agreement with the crystallite size from analysis of XRD of ca. 8 nm. This is the first time that NiCo2O4 has directly been produced using the continuous hydrothermal flow synthesis method. Magnetic susceptibility measurements were carried out on samples 5, 15, and 17-20. These measurements were used to calculate the effective number of unpaired electrons per molecule of the hydroxide; this was then plotted against the molar percentage of Co in the sample. The corresponding graph is shown in Figure 15. The general trend is for the number of unpaired electrons to increase as the amount of cobalt increases, since octahedral Co2+ has 3 unpaired electrons in the high spin state, whereas Ni2+ has only 2. The heavy line in Figure 15 shows the trend for an idealized NixCo(1-x)(OH)2 solid solution. The divergence of the experimental results from this line may be due to some of the Co being present in its trivalent state,62 in which it would only have two unpaired electrons. It is not

Phase pure Co3O4, CoO, Co(OH)2, CoO(OH), NiO, Ni(OH)2CoxNi(1-x)(OH)2 solid solutions, and NiCo2O4, respectively, were directly synthesized using a hydrothermal flow reactor using various additives such as KOH, H2O2, and PVP. For the cobalt compounds, the supercritical water feed temperature and concentration of added base were important in controlling which phase was produced. Peroxide and PVP were very effective in reducing particle sizes of nanopowders. Homogeneous CoxNi(1-x)(OH)2 solid solutions with the desired composition were obtained by simply adjusting the reagent metal feed composition. Heat-treatment of the mixed metal hydroxide solid solution with a 1:2 ratio of Ni/Co at 400 °C for 1 h in air yielded phase pure NiCo2O4 spinel as a nanopowder with minimal reduction in the surface area of the original material. Using relatively more severe conditions, we were also able to prepare this material directly in the hydrothermal flow system and with a very high surface area. Thus, the hydrothermal method is a highly controllable and versatile method for the manufacture of both doped and homo- and heterometallic nanomaterials. The speed and versatility of the continuous hydrothermal synthesis technique has also led our group toward an exciting new concept, namely, “high-throughput (semi)continuous hydrothermal nanoceramics manufacture”, in which many hundreds of nanomaterials can be sequentially generated using the hydrothermal technology; the results of these endeavors will be reported in due course. Acknowledgment EPSRC is thanked for funding an EPSRC Advanced Research Fellowship entitled “Next Generation Biomedical Materials Using Supercritical Fluids” (J.A.D.; Grant GR/A11304/01); postdoctoral research funding (A.K.H., Grant GR/S79183/01); the High Throughput Nanoceramics Discovery project [J.A.D.; Grant EP/D038499/1]; and an industrial case award (P.B.). Johnson Matthey is also thanked for supporting the industrial case award (P.B.). Dr. P. Blood and co-workers (University of Nottingham) are thanked for helpful discussions.

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ReceiVed for reView October 31, 2006 ReVised manuscript receiVed April 25, 2007 Accepted April 26, 2007 IE061396B