Single Step Synthesis of High-Purity CoO Nanocrystals - The Journal

Jun 23, 2007 - Citation data is made available by participants in Crossref's Cited-by Linking service. For a more comprehensive list of citations to t...
9 downloads 0 Views 692KB Size
Download PDF
8006

J. Phys. Chem. B 2007, 111, 8006-8013

Single Step Synthesis of High-Purity CoO Nanocrystals Huaming Yang,*,† Jing Ouyang,† and Aidong Tang‡ Department of Inorganic Materials, School of Resources Processing and Bioengineering, Central South UniVersity, Changsha 410083, China, and School of Chemistry and Chemical Engineering, Central South UniVersity, Changsha 410083, China ReceiVed: January 27, 2007; In Final Form: May 1, 2007

Both octahedral and slice-shaped cubic cobalt monoxide (CoO) nanocrystals with narrow size distributions have been successfully synthesized by a simple solvothermal route. It was found that conditions of the solvothermal treatment showed obvious effects on the formation and purity of the as-synthesized CoO nanocrystals, only when cobalt acetate was used as the cobalt source and when temperature reached 190 °C could CoO be produced; also, freeze-drying was necessary for obtaining pure CoO. Size of the CoO nanocrystals varied from 30 to 130 nm. Morphology of the products could be controlled by simply changing the type of surfactant in solvent, and the octahedral CoO nanocrystals showed rounded turns. Purity of the products was detected by intensive X-ray photoelectron spectroscopy (XPS) investigation and Fourier transform infrared spectroscopy (FTIR) combined with differential scanning calorimetry/thermal gravity (DSC/TG). The results indicated an absence of unexpected trivalence cobalt series on surface of the samples, thanks to the protection of the surface by trace amount of carbonate ions, adsorbed hydroxylation, and surfactant with a maximum thickness of 2 nm, which were proved by high-resolution transmission electron microscopy (HRTEM). The as-synthesized CoO nanoparticles were added into positive electrode of Ni/MH batteries, and discharge/ charge cycling tests were performed under different rates from 0.1C to 5.0C. The results indicated that the specific capacities of batteries with addition of 5% octahedral or slice CoO nanocrystals at 0.1C were 393.3 and 318.1 mAh/g, respectively, which were higher than that without CoO (269.2mAh/g). Specific capacity of battery with addition of 5% octahedral CoO nanocrystals was 40% higher than that without CoO at 5.0C. Octahedral CoO nanocrystals show better electrochemical activity than slice CoO and indicate interesting potential in the field of electrochemical application.

Introduction Cobalt monoxide (CoO) is an absolutely necessary additive in anode of Ni/H2 and Ni/Cd battery because it plays the role of enhancing the discharge deepness and enlarging the current, which are the two decisive factors in qualifying the energetic source with huge charge/discharge capacity, by drastically decreasing inner resistance of the battery through sediment onto surface of Ni(OH)2 particles and then transform into β-CoOOH.1 However, for taking full advantage of the energy storage material and for fully realizing the promising application prospects of CoO in fields of catalysts, gas sensors, magnetisms, giant magnetoresistance read heads, and media tapes,2-8 such unique properties as high purity, high density, high stability, ultrafine diameter, and monodispersion were required. It is foreseeable that the requests would be fulfilled if the oxide could be prepared into nanosize, as many unique properties of nanomaterials lie just in scope of the aforementioned requirements. CoO is a low-valence transition metal oxide which has for a long time been represented by dark yellow colore and raw salt fcc structure,9,10 although the structure will undergo a transition near its Neel temperature (TNeel ∼ 289 K),11 while a kind of dark green CoO with wurtzite-type hexagonal P63mc structure, which preciously was the represent of ZnO, was found * Corresponding author. Tel: +86-731-8830 549. Fax: +86-731-8710 804. E-mail: [email protected]. † School of Resources Processing and Bioengineering. ‡ School of Chemistry and Chemical Engineering.

recently.12-14 There already have some reports on the synthesis of nanosized CoO with different morphologies under the existence of certain capping reagents, such as sol-gel, electrochemical route, thermal decomposition of CoC2O4 for nanorods, pyrolytic Co2(SO4)3 in molten NaCl for CoO nanofibers.15-18 Nevertheless, some disadvantages may exist in the aforementioned methods, such as too many steps in the preparation process followed by expensive metal organic raw source consumption;a fatal fault might be ruled out that unexpected impurities such as Co3O4 would be formed when oxygen in the system has not been completely dispelled when the precursors are annealed in high quality vacuum or atmosphere controlling environment (such as nitrogen or argon protection). Complexity would be added into the synthesis process, and large-scale synthesis and application of this material would be limited. Therefore, to develop a simple and easycontrolling process for synthesis of nanoscale CoO is of great importance. Solvothermal method is a novel and promising method to prepare nanoparticles because it is a soft and environmentalfriendly route to reach hardy target by inducing complicated reactions, which may not take place in normal conditions, under the huge self-produced pressure by sub- or supercritical solvents. Some single-step processes have been developed under solvothermal conditions such as chemical decomposition of Co(II) cupferronate in Decalin19 or cobalt acetylacetonate (Co(acac)2) in certain reaction media to prepare CoO nanospheres; following this avenue, monodispersed CoO nanocrystals in size of

10.1021/jp070711k CCC: $37.00 © 2007 American Chemical Society Published on Web 06/23/2007

High-Purity CoO Nanocrystals 13-33 nm and 4.5-23 nm were prepared by Seo et al.12 and Zhang et al.,20 respectively. In addition, Yin and Wang21 presented a novel magnetic field phase-selection technique to prepare high purity tetrahedral CoO nanocrystals with edge length of 4.4 nm. Our effort in this article involves taking advantage of the excellence of solovothermal method to develop a cheap, single-step as well as shape-controlling route for preparing CoO nanocrystals using cobalt acetate as the precursor. The process of choosing the precursor will be presented. The results indicated that our experiments yielded high purity CoO slices and nano-octahedra. Experimental CoO Nanocrystal Synthesis. The cobalt sources and ethanol and organic surfactants were used without further purification. In typical synthesis process, 1 mmol of different kinds of cobalt sources, including cobaltous chloride (CoCl2‚6H2O), cobalt acetate (Co(Ac)2‚4H2O), cobaltous carbonate hydroxide (Co5(OH)6(CO3)2‚nH2O), and cobaltous hydroxide (Co(OH)2, obtained from the precipitation of CoCl2‚6H2O by ammonia), was dispersed or dissolved in pure ethanol under supersonic vibration to form a mixture. For the purpose of evaluating the effection of different surfactants on morphology of the products, either cetyltrimethyl ammonium bromide (CTAB) or polyvinyl pyrrolidone (PVP) was also added into the mixture. The mixture was then put into a Teflon-lined stainless steel autoclave which was filled up to 75% of its total volume. The autoclave was treated at different temperatures from 130 to 220 °C, and maintained for different long time from 2 to 8 h before cooled in air. The fulvous products were washed both by ethanol and deionized water, and finally dispersed in deionized water for further characterization. XRD Characterization. For XRD detection, the as-prepared dispersion was frozen at -40 °C by a Sanyo MDF-V333 biomedical freezer, and then lyophilized by a LGJ-10 freeze drying machine at -53 °C under pressure of 18 Pa. The samples were characterized by a RIGAKU D/max-2550VB+ 18kW powder diffractometer with Cu KR-radiation (λ ) 1.541806 Å). Data were collected from 10° to 90° of 2θ with a step width of 0.02°. Phases were identified using the Search/Match capabilities of the JADE 5.0 program along with the ICDD (International Center for Diffraction Data) powder diffraction file (PDF) database. Sizes of the nanocrystals were calculated according to Scherrer’s equation based on half-width of CoO (200) diffraction peak. SEM and HRTEM Characterization. Morphology and lattice image of the nanocrystals were detected by FEI Sirion 200 field emission scanning electron microscopy (SEM) and JEM-2010 high-resolution transmission electron microscopy (HRTEM) equipped with Gatan-794MSC CCD selected area electron diffraction photographer (SAED, the acceleration voltage was 200kV), Gatan-676TV system and OXFORD-INCA super atmospheric thin window energy dispersive X-ray spectroscopy (EDS). SEM sample was prepared by dropping the as-prepared dispersion onto brass stubs and dried under vacuum at room temperature through evaporation; the stubs were then conductively coated with gold by sputtering for 10 s to minimize charging effects under SEM imaging. The sample for TEM investigations was prepared by dropping the as-prepared dispersion onto an amorphous carbon substrate supported on a 300 mesh copper grid, then tried along with the aforementioned method. XPS and TG/DSC Characterization. The X-ray photoelectron spectroscopy (XPS) experiments were performed on a VG

J. Phys. Chem. B, Vol. 111, No. 28, 2007 8007 MK-II spectrometer equipped with an Al KR (1486.6 eV) monochromator X-ray source running at 3 kV, a hemispherical electron energy analyzer, and a multichannel detector. The test chamber pressure was maintained below 2 × 10-9 Torr during spectral acquisition. A low-energy electron gun was used to neutralize the possible surface charge. The XPS binding energy (BE) was internally referenced to the C 1s peak (BE ) 284.6 eV). Survey spectra were acquired at step of 0.4 eV, while high-resolution spectra were acquired at step of 0.05 eV. The analyzer pass energy was set as 100 eV. The takeoff angle was defined as the angle between the surface normal and detector. High-resolution spectra were resolved by fitting each peaks with Gaussian-Lorentz functions after subtracting the background using the ESCALAB V1.5 data processing software package under the constraint of setting a reasonable BE shift and characteristic full width at half-maximum range. Atomic concentrations were calculated by normalizing peak areas to the elemental sensitivity factor data provided by the ESCALAB database. Thermal gravity and differential scanning calorimetry (TG/DSC) test of the freeze dried sample was carried out on a NETZSCH STA449C thermal analyzer. 30 mg of the powder was filled into a corundum pot and a same weighted R-Al2O3filled pot was used as the reference. An electronic scale and a calorimetry measure were introduced as the detector, baseline of the DSC curve and the measured temperature was adjusted by calorimetric curves and melting point of standard indium metal. FTIR and Electrochemical Characterization. Fourier transform infrared (FTIR) absorption spectrum of the freeze-dried sample was detected by a Nicolet NEX-US 670 IR spectroscopy, analytical-grade KBr was used as the dispersant, the range of the spectrum was settled in 400-4000 cm-1, and data were scanned repeatedly for 32 times and then normalized. Electrochemical tests were conducted using the self-made model cells . The positive electrode and negative electrode were constituted from 85% commercially purchased β-Ni(OH)2 with addition of 4% ZnO as the cathode material, or 85% commercially purchased AB5-type hydrogen powder as negative materials, 10% acetylene black as conducting additive, and 5% polytetrafluoroethylene as binder. In order to compare with the electrochemical activity of the cathode materials, our as-prepared CoO nanoparticles were added into the cathode materials with the amount of 5%. It was dried at 60 °C for 24 h in a vacuum oven to remove moisture and pressed onto a 1 cm2 foam nickel mesh under pressure of 20 MPa. The specific capacity of the negative materials was assured to overdose at least 2 times than that of the cathode materials. The 6 mol/L KOH solution was used as the electrolyte. The cells were cycled galvanostatically in the voltage rang 0.9-1.5 V at a desired current density at 25 °C with a LAND battery Program-Controlling Tester System (China). The cells were activated by cycling 4 times at a small current about 10 mA before testing. Results and Discussion It was found that raw salt structured CoO from the relatively cheap and easily reached cobalt (II) acetate (Co(AC)2) was reasonable, although it was reported by Risbud et al.13 that the precursor was not suitable for preparing hexagonal CoO in their efforts. In addition, Yin et al.22 illustrated a novel and so-called “TDMA” (thermal decomposition of metal acetates) method which was believed to be very effective for the synthesis of first-row transition metal and transition metal oxide nanocrystals. Furthermore, synthesis of transition metal oxide nanocrystals by choosing their acetates as precursors revealed to be simple and relatively safe according to their research.23,24 The results

8008 J. Phys. Chem. B, Vol. 111, No. 28, 2007

Yang et al.

Figure 1. XRD patterns of (A) products from different raw chemicals and (B) pure CoO.

of our efforts complied with their proposal, too. XRD patterns of products from different cobalt sources were shown in Figure 1A, and detailed conditions and results for curves in Figure 1A were listed in Table 1. It can be learned from Table 1 that only when cobalt acetate was selected as the Co source could the target CoO came into being after hydrothermal/solvothermal treatment, cobalt oxide (Co3O4) or cobalt chlorine hydroxide, even uncompleted decomposition of raw source were got when the other cobalt sources were chosen. The detailed XRD peaks of the ideal product were shown in Figure 1B, all of the peaks match well with Bragg reflections of the standard raw salt cubic CoO structure (Fm3hm, a)4.261 Å, Joint Committee on Powder Diffraction Standards File No. 481719). Lattice parameter of the sample based on CoO (200) peak is 4.269 Å, in a good agreement with the standard value. Diffraction peaks of the sample are sharp and narrow, indicative of a relatively large crystal size, and the value was revealed to be 52.5 nm according to calculation by Scherrer’s equation.

It was worth notifying that method for drying the dispersion did have great effect on purity of the final product, the dark yellow dispersion will be converted into black powders when it was dried in a medical vacuum chamber at 60 °C, indicating the existence of large amount of impurity of Co3O4(Fd3m, a ) 8.084 Å, JCPDS No. 43-1003). But the color of the resulted powder kept in line with original ones when the same dispersion was dried under lyophilizes process and pure CoO was obtained according to XRD result, indicating that temperature and oxygen pressure of the chamber take great responsibility for stability of the CoO nanocrystals; the much higher O2 concentration and environmental temperature in vacuum chamber than in the freeze drying machine would accelerate the cobalt (II) oxidizing into cobalt (III). The extremely easy conversion of the products can be reasoned by the structure similarity of the two oxides. The rocksalt monoxide are not the only binary oxide phase which is formed under readily attainable oxygen partial pressures for transitional metal with partially filled 3d orbital under common ambient conditions. The thermal dynamically favored form of the oxide often is the spinel M3O4 (M ) Mn, Fe, Co, etc., with the exception of Ni),25 even when mass transport limits the bulk of the oxide in a metastable monoxide form, the surface often contains significant amount of spinel phase.26 For cobalt series, both rocksalt CoO and spinel Co3O4 have a comparable oxygen sublattice with fcc closest packed structure in which O2--O2nearest-neighbor distance match to within 5%.27 In the monoxide, Co2+ cations occupy all octahedral sites, whereas in the Co3O4, the spinels feature two metal oxidation states (Co2+/Co3+) and two different cation sites (tetrahedral/ octahedral), each type of which is only partially occupied to give a unite cell of 56 atoms; half of the octahedral sites are populated with Co3+, and one-eighth of the tetrahedral sites are populated with Co2+ in a specific, well-ordered manner.28 Under oxidizing conditions, sufficient mobility of the cobalt cations in the near surface region is also permitted. That means the cobalt cations who should be oxidized would keep in their octahedral positions, and only those Co(II) cations should be transferred into the tetragonal sites. As a result, CoO is apt to spontaneously transform into spinel Co3O4 when it stands in the circumstance even with low oxygen partial pressure if no protection should exist. It was widely accepted that the capping reagents is very important in wet synthesis of nanocrystals during nucleation, growth, and stabilization of the particles, and it is also important in postsynthesis processes when agglomeration and stabilization in organic solvents are concerned. Although the detailed mechanisms of the cooperation were not clear, it was firmly believed that different kinds of surfactants may result in different kinds of morphologies of the products through affecting the surface energy and developing orientation of crystal facets and through affecting the polarity and Van der Waals forces between the long chain molecules and crystal facets.29 Either PVP or CTAB was introduced here to realize the purpose of shape

TABLE 1: Typical Products from Different Raw Materials label in Figure 1

cobalt source

solvent

calcination temperature (°C)

calcination time (h)

products according to XRD detection

a b c d e f g h

Co(OAc)2‚4H2O Co(OAc)2‚4H2O Co(OAc)2‚4H2O Co(OH)2 CoCl2‚6H2O + NaOH CoCl2‚6H2O + NaOH CoCl2‚6H2O + NaOH Co5(OH)6(CO3)2‚nH2 O

enthanol enthanol enthanol H2O enthanol 50% H2O + 50% enthanol H2O H2O

200 200 200 200 170 170 150 130

6 6 4 4 4 4 4 4

CoO+Co3O4 CoO CoO + undefined Co(OH)2 + Co3O4 Co2(OH)3Cl + Co3O4 Co2(OH)3Cl Co(OH)2 raw + Co3O4

High-Purity CoO Nanocrystals

J. Phys. Chem. B, Vol. 111, No. 28, 2007 8009

Figure 2. SEM images of product from CTAB as dispersant under 190 °C for 16 h (A, B) and from PVP as dispersant under 220 °C for 8 h (C, D).

Figure 3. (A) TEM image of the PVP capped sample and (B) size distribution for the CoO nanoparticles.

controlling the products for different applications. The SEM images of products from cobalt acetate under thermal treatment of 200 °C after 16 h capped by CTAB and that capped by PVP at 220 °C for 8 h are shown in Figure 2. As can be observed from Figure 2, where CoO slices appeared when CTAB was used as the capping reagent and the precursor was treated at 200 °C for 16 h, the average diameter of the CoO sheets is about 150 nm, obviously larger than that calculated from XRD data. However, monodispersed particles with perfect octahedral shape were obtained when PVP was quoted as the capping reagent; the size of these octahedra is about 80 nm, fairly close to that from XRD results. TEM image of the sample capped by PVP is shown in Figure 3A, and the corresponding crystal size distribution is illustrated

in Figure 3B by counting 100 particles in the visual field. It can be considered that size of the particles mostly ranged from 60 to 90 nm, which perfectly complies with the SEM results, although there is a little part of the crystals larger than 100 nm. The intensive study for profile of the nanocrystals is shown in Figure 4, which reveals that the highly crystallized nanosrystal has a four square fundus on which symmetric pyramid developed; the shape of the crystal was modeled in Figure 4H, and the octahedral shape is absolutely different from the result of Seo et al.,12 who obtained the pyramids though no mirror structure developed, which may be attributed to the difference of symmetry of the crystals, as cubic structure is more symmetric than hexagonal one. Furthermore, turns and angles of the nanocrystals are slightly rounded rather than normally recog-

8010 J. Phys. Chem. B, Vol. 111, No. 28, 2007

Yang et al.

Figure 4. HRTEM images of (A-E) some of the nanocrystals; (F) amplified turn in crystal (E); (G) lattice fringes of the surface; (H) modeled profile for the nanocrystals and indexed SEAD photography; (I) EDS spectrum of the selected area; and (J) TG/DSC curves of the sample.

nized as sharp ones, as can be seen in Figure 4E,F. The 2-D lattice fringes in Figure 4G illustrate that the nanocrystals are perfectly developed single crystallite with no defects or dislocations. Interplanar spacings were ∼2.46 Å, indicating the (111) planes. Energy dispersion spectrum (EDS) of the selected area in Figure 4I indicates that these nanocrystals are composed mostly of O and Co (as element C and Cu are the evidence of amorphous carbon covered copper grid). Selected area electron diffraction (SAED) patterns (Figure 4H) can be indexed to the reflection of cubic symmetric with the lattice parameter of a)0.4265 nm, confirming the cubic crystal structure of CoO. The different shape of products obtained from different circumstances may be explained by combination of thermodynamic aspects of crystal growth and the selective-adsorption model of surfactants (capping reagents) on different crystallographic facets.30 For crystals grown under thermodyn amically balanced conditions, just like our experimental conditions, their shapes of the crystals are determined by the surface free energy of individual crystal facets, and their final morphology will be determined by the competition of minimizing the total free energy of the system. In most conditions, the product is composed of those facets whose developing rate are relatively slow.31 Accounting for this theory, the lowest surface energy and atom density of (111) planes within the three lowest index planes ((111), (200), and (110)) in cubic structure may take the most responsibility for (111) dominating the orientation in the crystal-growth processes.32 The selective-adsorption model fits both to mixed surfactants and to monosurfactant systems. According to this famous model, the surfactant molecules selectively adsorbed onto certain faces of the growing crystals, resulting in modification of surface free energy of these facets and leading to the crystals grown along these planes; the different developing directions of crystallographic faces result in the controlled shape of products. In the case of the octahedral-

shaped sample, it my be deduced that development of the (111) facets was handicapped by the adsorbed PVP surfactant on these facets, which will be further verified by the following investigation, thus allowed the crystals develop into a shape whose exposed planes were (111) ones. It is worth noticing that there exists an average thickness of about 2 nm amorphous component on surface of the as-prepared nanocrystals, as labeled A in Figure 4F, suggesting a certain dose of surfactant adsorbed on surface of the nanocrystals, which is the partial reason why our nanosized CoO can retain its stability in air, as its surface has been protected by the organic molecule. This evidence of surfactant adsorption can be further verified by TG/DSC analyses and intense investigation of the C1s electron spectrum in XPS and FTIR spectrum. The TG curve of the sample shown in Figure 4J, illustrating a main mass decrease (8.9%) before 320 °C when the heating rate was 8 K‚min-1, indicating the decomposition of adsorbed surfactant, followed by a relatively slight stage of mass increase (3.8%) on the curve due to the oxidation of CoO, although the percentage of the mass increase was smaller than the theoretical value (7.1%). The divergence can be reasoned by partial oxidation of the product in early stage of the heating, as even a slight temperature increase in the vacuum drying process could result in the slow transformation of the sample (which has been described in the XRD discussion), let alone when temperature uplifted to 320 °C. The DSC curve of the sample displayed two exothermal peaks in the gravimetric loss region: one is centered at 245 °C and another at 310 °C, indicating at least two kinds of substance accompanied with the nanoparticles. According to the core-level XPS spectrum of O 1s electrons of the sample, the peak at 310 °C was attributed to the combustion of adsorbed surfactant on surface of the particles, and the other one to desorption of surface hydroxide. The immense peak from 400 to 700 °C is

High-Purity CoO Nanocrystals

J. Phys. Chem. B, Vol. 111, No. 28, 2007 8011

Figure 5. Full survey XPS spectra of CoO sample (a) from PVP capping and (b) from CTAB capping. The inserted profile is the C 1s survey curve with the fitting curves.

Figure 6. Co 2p electron XPS spectra and its fitting curves for (a) sample from PVP capping and (b) sample from CTAB capping.

regarded as the shift of DSC baseline based on many detections carried out on the same instrument. Surfaces of the nanosized materials are much vulnerable to being affected by the circumstance because of their special immense surface area and high activity. The cubic 3d transitionmetal Co has a partially filled valence band within which strong electron correlations among the 3d electrons were localized.33 It is of great importance to study the stability and status of atoms on surface of the as-prepared nanocrystals, considering the special requirements rising from electrochemical application of CoO. X-ray photoelectron spectroscopy (XPS) is a reliable method for intensive investigation of nanosized samples, especially efficient for studying the statues of atoms deep into a few layers on surface of metastable materials with partially filled valence band. It was introduced here to evaluate the surface of the as-prepared CoO nanocrystals both from PVP capping and from CTAB capping. The full survey spectra of the two samples are just in coherence with each other, which were shown in Figure 5, where each of the main peaks are indexed to O 1s, C 1s and Co 2p regions, confirming nonexistence of metallic contaminates. The inserted profile is the banding energy (BE) spectrum for C 1s electrons of PVP capped sample. The curves are fitted by Gaussian-Lorentz functions; a main peak emerged on the fitting line at BE of 284.6 eV, which is the standard BE for the amorphous carbon as inert reference, yet there has a relatively mean peak appeared at BE of 286.6 eV, suggesting that a carbon impurity exists except the amorphous one, which can be regarded as another evidence of the surfactant adsorption on surface of the nanocrystals. The BE of organic carbon atoms should more or less increased compared with the amorphous one, as the BE of an element is determined by chemical environments around it. Carbon in organics are surrounded mainly by hydrogen and meanly by oxygen, nitrogen, and so on, which will for certain degree increase the electron vacancies in the valence shells of carbon atoms compared with the amorphous one, and therefore increase the electrical contracting forces between nucleus and electrons in inert shells, finally resulting in the increasing of BE for inner electrons. Cobalt series occupy the identical BE region belonging to Co 2p1/2 and Co 2p3/2 electrons, but the distinction among many of the cobalt compounds is still vague, as chemical shift of main peaks in XPS spectra of Co 2p electrons in Co2+ and Co3+ is not obvious enough. The supposed BE values of cobalt 2p3/2 and 2p1/2 were 780.5 and 796.5 eV for the monoxide and 780.5 and 795.7 eV for the spinel.33 Nevertheless, identical satellite

structures apart from the main peaks are evident of Co(II) existence.20,34-37 The satellite should be attributed to shakeup lines and resulted from the charge-transfer band structure of the late 3d transition metal monoxide. Intensity of both satellite and 2p1/2 lines will decrease immensely when spinel structure takes the domination, which has been previously confirmed by intensive XPS and Auger emission spectroscopy (AES) studies for Co 2p electrons in epitaxial grown Co3O4 on CoO (100) and for Li-doped CoO and LiCoO2.37,38 Furthermore, other methods were introduced to confirm the monoxide existence. For example, the ratio for intensity of O 1s/Co 2p XPS and Auger O KL2L2 /Co L3M4,5M4,5 increase by approximately 4/3 would reflect the change from monoxide to spinel stoichiometry.37 Intensity ratio of Co 2p1/2 satellite to main peak (abbreviated as value c1 in this paper) equals ∼0.9, characteristic of the cubic CoO, and ∼0.3 for Co3O4.39 Intensity ratio of XPS O1s/Co2p (using the entire Co 2p1/2 that is satellite plus main peak, abbreviated as value c2) was recommended as 0.45 in the literature.40 The survey spectra for Co 2p region of the two samples are shown in Figure 6. The two curves are just in the same shape. A main peak centered at 780.1 eV, which is the identical BE of Co 2p3/2 electrons, and a relatively weaker peak at BE of 796.0 eV, which is the identity of Co 2p1/2 electrons, appeared on the curves. Obvious shakeup lines were observed at 785.3 and 802.6 eV, which should confirm the CoO stabilization in our samples. The deconvolution fitting lines of the peaks perfectly analogue with the original data and no additional Gaussian peaks exist on the narrow region survey curves, indicating no evidence of cobalt metal existing in the samples. However, on considering the chemical valence disunity of the cobalt cations and the uncertainty of Co 2p XPS spectra, intense study should be carried out for confirming the experimental results. Previous studies have shown that the lattice O 1s BE for transition metal monoxide was relatively insensitive to changes in near-surface stiochiometry, but the O 1s peak due to lattice oxygen in CoO was assigned to a value of 529.4 eV as an internal BE calibration.34 Carson and coauthors37 showed the appearance of a second oxygen component at 531.2 eV in the O 1s data after exposure to the CoO surface into 5 × 10-7 Torr O2 at 635K for 30 min; the feature continued to increase slowly in intensity to saturate after approximately 300 min. The 531.2 eV peak of O 1s region has been attributed to nonstoichiometric near-surface oxygen, a more specific near-surface O-, oxygen atoms in carbonate ions (which are disposed on

8012 J. Phys. Chem. B, Vol. 111, No. 28, 2007

Yang et al.

Figure 7. O 1s XP spectrum and its fitting curves of sample from PVP capping. Figure 9. Specific capacities for Ni/MH batteries with different CoO nanocrystals.

Figure 8. FTIR spectrum of the PVP-capped sample.

surface of CoO), surface hydroxylation and Co(OH)2-like thin film formation,41-44 as contracting forces between oxygen atom nuclei and its inner electron layers will increase in case of the higher positively charged cations, such as cobalt (III) compared to cobalt (II), or higher electronic condensed cations, for the hydrogen ions as an example, surrounding the oxygen atoms. As this will inevitably increase the static electrical forces between the cations and negatively charged external electrons in oxygen atoms and result in the uplift of BE in internal electron shells of oxygen. O1s XPS spectrum for one of the two samples (as the other one is just as similar) was shown in Figure 7. A major peak at BE of 530.1 eV and a mean peak at 532.1 eV were observed by Gaussian curve fitting resolution, suggesting that at least two kinds of chemical environments exist in oxygen atoms of the sample. The mean peak was ascribed to the hydroxyl and carbonate ions on surface of the sample, despite of the upshift of the BE value for oxygen in CoO and in the as-ascribed -OH and CO32- groups. The ascription was concluded from the exothermal peak appeared at 245 °C and from the following discussions: As BE of oxygen in hydroxyl and that in carbonate ions cannot be clearly distinguished from each other, but their adsorption to infrared wave can be fully divided due to their totally different structures, FTIR spectrum was used to detect the surface groups on the as-synthesized sample, and the result is shown in Figure 8. The absorbance located around 420 cm-1 is the stretching vibration of Co-O bonds, and the main band at around 3430 cm-1 is evidence of -OH stretching deformation. Another band at 665 cm-1 and the broad one in range of 1225-1734 cm-1 should belong to the IR absorption of the

absorbed PVP surfactant.45-48 The weak bands located at 830 and 1050 cm-1 are representations of out-of-plane deformation (V2) and asymmetric stretching (V3) of carbonate ions, respectively,49 which may be formed in process of solvothermal treating of the mixture of raw sources and surfactants. Intensities of the band belonged to CO32- are much weaker than those of the other bands, indicative of the much smaller magnitude of the composition. In addition, the pre-mentioned c1 value based on ratio of area under fitting curve of Co 2p1/2 satellite and its main peak is 0.87, and the value c2 based on ratio between area under O 1s fitting structure centered at 530.1 eV and the entire Co 2p1/2 region (that is satellite plus main peaks area) is 0.49. The two are all in good agreement with the literature recommended values, suggesting that the CoO in the experiment contains no contamination developed from cobalt series. Therefore, the O 1s peak centered at 532.1 eV should not be ascribed to the nonstoichiometric near-surface cobalt oxides, but mainly to the surface hydroxylation and meanly to the carbonate ions, as the resonance to result of DSC detection. The as-synthesized CoO nanoparticles were added into positive electrode of Ni/MH batteries and discharge/charge cycling tests were carried out under different rates from 0.1C to 5.0C. Obvious specific capacity enhancement was obtained from Figure 9. The results indicated that the specific capacity of battery with addition of 5% octahedral CoO nanocrystals at 0.1C was 393.3 mAh/g, which was higher than that without CoO (269.2mAh/g). The battery with addition of 5% slice CoO nanocrystals also illustrated a discharge enhancement (318.1mAh/ g) than that without CoO, although the effect was not so obvious than that doped by octahedral particles. This possibly can be reasoned by the smaller size of the octahedral CoO nanoparticles than the slice CoO, as smaller size suggests a larger surface area and better attachment onto the Ni(OH)2 surface. Otherwise, specific capacity of battery with addition of 5% octahedral CoO nanocrystals was 40% higher than that without CoO at 5.0C. So, octahedral CoO nanocrystals show interesting application potential in the electrochemical application. Conclusions In summary, rock salt CoO nanocrystals with octahedral or slice shape were synthesized by a single-step solvothermal route. The process yielded CoO nanocrystals with different morphologies, which were controlled by simply changing the kind of surfactant. Intensive study on the PVP capped sample shows that the octahedral CoO crystals have rounded turns. It is

High-Purity CoO Nanocrystals revealed that the products contained no unexpected cobalt series impurity, because of the coverage of adsorbed surfactant and hydroxylation and trace amount of carbonate ions on surface of the CoO nanocrystals, which were proven by both thermal and XPS analysis and thickness of the coverage was observed by HRTEM image to be a maximum of about 2 nm. The application tests suggested an obvious enhancement of the specific capacity of the Ni/MH batteries, indicating that the single step prepared CoO nanocrystals with ultrahigh purity and ultra fine diameter significantly met the requirements for electrochemical application of CoO nanomaterials. Acknowledgment. This work was supported by the Program for New Century Excellent Talents in University (NCET-050695) and the Program for New Century 121 Excellent Talents in Hunan Province (05-030119). We thank Dr. Guangshe Li (State Key Lab of Structural Chemistry, Fujian Institute of Research on the Structures of Matter, Chinese Academy of Science) for kindly helping in HRTEM detecting. We thank Dr. Hong Zhang (School of Materials Science and Engineering, Central South University, Changsha, China) for her helping in SAED analysis. References and Notes (1) Oshitani, M.; Yufu, H.; Takashima, K.; Tsuji, S.; Matsumaru, Y. J. Electrochem. Soc. 1989, 136, 1590. (2) Logothetis, E. M.; Park, K.; Meitzler, A. H.; Laud, K. R. Appl. Phys. Lett. 1975, 26, 209. (3) Koshizaki, N.; Yasumoto, K.; Sasaki, T. Nanostruct. Mater. 1999, 12, 971. (4) Thorma¨hlen, P.; Skoglundh, M.; Fridell, E.; Andersson, B. J. Catal. 1999, 188, 300. (5) Mocuta, C.; Barbier, A.; Renaud, G. Appl. Surf. Sci. 2000, 162163, 56. (6) Pan, T.; Spratt, G. W. D.; Tang, L.; Laughlin, D. E. J. Magn. Magn. Mater. 1996, 155, 309. (7) Raquet, B.; Mamy, R.; Ousset, J. C.; Ne`gre, N.; Goiran, M.; Guerret-Pie´court, C. J. Magn. Magn. Mater. 1998, 184, 41. (8) Daniel, M. R.; Cracknell, A. P. Phys. ReV. 1969, 177, 932. (9) Rechtin, M. D.; Averbach, B. L. Phys. ReV. Lett. 1971, 26, 1483. (10) Khowash, P. K.; Ellis, D. E. Phys. ReV. B 1987, 36, 3394. (11) Rechtin, M. D.; Averbach, B. L. Phys. ReV. B 1972, 5, 2693. (12) Seo, W. S.; Shim, J. H.; Oh, S. J.; Lee, E. K.; Hur, N. H.; Park, J. T. J. Am. Chem. Soc. 2005, 127, 6188. (13) Risbud, A. S.; Snedeker, L. P.; Elcombe, M. M.; Cheetham, A. K.; Seshadri, R. Chem. Mater. 2005, 17, 834. (14) Grimes, R. W.; Fitch, A. N. J. Mater. Chem. 1991, 1, 461. (15) Zhang, L.; Xue, D. J. Mater. Sci. Lett. 2002, 21, 1931. (16) Reddy, E. P.; Rojas, T. C.; Sachez-Lopez, J. C.; Domı´nguez, M.; Rolda´n, E.; Ca´mpora, J.; Palma, P.; Ferna´ndez, A. Nanostruct. Mater. 1999, 11, 1261. (17) Xu, C.; Liu, Y.; Xu, G.; Wang, G. Chem. Phys. Lett. 2002, 366, 567.

J. Phys. Chem. B, Vol. 111, No. 28, 2007 8013 (18) Zhan, Y.; Yin, C.; Wang, W.; Wang, G. Mater. Lett. 2003, 57, 3402. (19) Ghosh, M.; Sampathkumaran, E. V.; Rao, C. N. R. Chem. Mater. 2005, 17, 2348. (20) Zhang, H.-T.; Chen, X.-H. Nanotechnology 2005, 16, 2288. (21) Yin, J. S.; Wang, Z. L. Phys. ReV. Lett. 1997, 79, 2570. (22) Yin, M.; Wu, C.-K.; Lou, Y.; Burda, C.; Koberstein, J. T.; Zhu, Y.; O’ Brien, S. J. Am. Chem. Soc. 2005, 127, 9506. (23) Yin, M.; Gu, Y.; Kuskovsky, I. L.; Andelman, T.; Zhu, Y.; Neumark, G. F.; O’Brien, S. J. Am. Chem. Soc. 2004, 126, 6206. (24) Yin, M.; O’Brien, S. J. Am. Chem. Soc. 2003, 125, 10180. (25) Langell, M. A.; Kim, J. G.; Pugmireb, D. L.; McCarroll, W. J. Vac. Sci. Technol., A 2001, 19, 1977. (26) Oku, M.; Sato, Y. Appl. Surf. Sci. 1992, 55, 37. (27) Shannon, R. D.; Prewitt, C. T. Acta Crystallogr., Sect. B 1969, 25, 925. (28) Wang, J. Low Energy Electron Diffraction Studies of Transition Metal Oxide Surfaces and Films, Martin-Luther-Universita¨t-Halle-Wittenberg, 2005. (29) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (30) Kumar, S.; Nann, T. Small 2006, 2, 316. (31) Mullin, J. W. Crystallization, 4th ed.; Butterworth Heinemann: Boston, 2001. (32) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (33) Langell, M. A.; Anderson, M. D.; Carson, G. A.; Peng, L.; Smith, S. Phys. ReV. B 1999, 59, 4791. (34) Kim, K. S. Phys. ReV. B 1975, 11, 2177. (35) Shen, Z. X.; Allen, J. W.; Lindberg, P. A. P.; Dessau, D. S.; Wells, B. O.; Borg, A.; Ellis, W.; Kang, J. S.; Oh, S. J.; Lindau, I.; Spicer, W. E. Phys. ReV. B 1990, 42, 1817. (36) Jime´nez, V. M.; Ferna´ndez, A.; Espino´s, J. P.; Gonza´lez-Elipe, A. R. J. Electron. Spectrosc. Relat. Phenom. 1995, 71, 61. (37) Carson, G. A.; Nassir, M. H.; Langell, M. A. J. Vac. Sci. Technol., A 1996, 14, 1637. (38) J. van. Elp; Weiland, J. L.; Eskes, H.; Kuiper, P.; Sawatzky, G. A.; Groot, F. M. F. de; Turner, T. S. Phys. ReV. B 1991, 44, 6090. (39) Grimblot, J.; Bonnelle, J. P.; Beaufils, J. P. J. Electron. Spectrosc. Relat. Phenom. 1976, 8, 437. (40) Scofield, J. H. J. Electron. Spectrosc. Relat. Phenom. 1976, 8, 129. (41) Lee, B. W.; IgnatievJ, A.; Taylor, A.; Rabalais, J. W. Solid State Commun. 1980, 33, 1205. (42) Klingenberg, B.; Grellner, F.; Borgmann, D. Surf. Sci. 1993, 296, 374. (43) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, 1979. (44) Tyuliev, G.; Angelov, S. Appl. Surf. Sci. 1988, 32, 381. (45) Borodko, Y.; Habas, S. E.; Koebel, M.; Yang, P.; Frei, H.; Somorjai, G. A. J. Phys. Chem. B 2006, 110, 23052. (46) Yang, Y.; Wang, W.; Li, J.; Mu, J.; Rong, H. J. Phys. Chem. B 2006, 110, 16867. (47) Wang, L.; Cui, S.; Wang, Z.; Zhang, X.; Jiang, M.; Chi, L.; Fuchs, H. Langmuir 2000, 16, 10490. (48) Qin, S.; Qin, D.; Ford, W. T.; Herrera, J. E.; Resasco, D. E. Macromolecules 2004, 37, 9963. (49) Pe´rez-Ramı´rez, J.; Mul, G.; Kapteijn, F.; Moulijn, J. A. J. Mater. Chem. 2001, 11, 2529.