CrystEngComm PAPER. Formation of shape-selective magnetic cobalt oxide nanowires: environmental application in catalysis studies.

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1 PAPER Cite this: CrystEngComm, 2013, 15, 482 Formation of shape-selective magnetic cobalt oxide nanowires: environmental application in catalysis studies Subrata Kundu,* a M. D. Mukadam, b S. M. Yusuf b and M. Jayachandran a Received 28th August 2012, Accepted 30th October 2012 DOI: /c2ce26382c A new route for the formation of shape-selective CoO nanowires has been developed using a simple microwave (MW) heating method. The reduction of Co(II) ions was done using a new reducing agent alkaline 2,7-dihydroxy naphthalene (2,7-DHN) in cetyltrimethylammonium bromide (CTAB) micellar media. The reaction mixture was irradiated using MW for a total time of 6 min. The process exclusively generates CoO nanowires of different lengths and having diameter y5 2nmto15 2 nm range just by tuning the metal-ion-to-surfactant molar ratios and changing the other reaction parameters. Magnetization measurements indicate that there is no observable coercivity for the short nanowires, but the coercivity increases as the length of the nanowires increases although the magnetic moment values at the maximum applied magnetic field of 2 T decreased with an increase in the length of the nanowires. The synthesized CoO nanowires are found to serve as an effective catalyst for the mineralization of several organic dye molecules in the presence of NaBH 4 in a short reaction time. The process assists the room temperature mineralization of the dyes and provides a cleanup measure of dye contaminated water bodies even in the presence or in the absence of light. The yield of the CoO nanowires with uniform shapes is found to be significantly high (.95%) and the nanowires are stable for more than a month under ambient conditions. The proposed synthesis method is efficient, straightforward, reproducible, and robust. Other than in catalysis, the cobalt oxide nanomaterials can also be applied for making pigments, lithium-ion battery materials, solid state sensors, or as anisotropy source for magnetic recording. Introduction Over the past few years, nanoparticles (NPs) research has become an intense focus due to their unique properties and potential applications. Nanomaterials having nanometer scale dimensions exhibit special properties compared to that of their bulk materials because of a high surface to volume ratio and a proportionally larger number of atoms at the surface. At nanoscale, their optical, 1 electronic, 2 magnetic, 3 and catalytic 4 properties change and all these properties mostly depend upon their size and shape. Among the various metal and metal oxide NPs studied so far, the transition metal oxides are an important group of materials as they form a wide variety of structures, display many interesting properties, and have numerous applications. They also show fascinating color change in the UV-vis region due to their close lying conduction and valence bands in which electrons can move freely. This fascinating color change of these materials depends upon both on their size and shape as well as on the refractive index of the surrounding medium. Among the various transition metal oxide nanocrystals, cobalt oxides are the subject of considerable interest because of their interesting fundamental properties and technological applications. In particular, CoO nanostructures can be used as catalyst precursors, to prepare pigments, conducting inks, multilayer capacitor, magnetic recording media, magnetic fluids, computer hard disks, lithium-ion battery materials etc. due to their superior chemical stability and magnetic properties Cobaltous oxide typically crystallizes in two stable phases, one in the cubic rock salt CoO with octahedral Co 2+ and the other in the hexagonal wurtzite CoO with tetrahedral Co 2+ ions. 10 There are numerous reports for the preparation of CoO particles but it is important to mention that preparation of pure CoO is difficult to achieve by a simple chemical route as it might produce CoO with a small amount of Co 3 O 4 and Co metal. Moreover, synthesis of size and shape controlled CoO NPs is a very tedious task, because in most of the cases it implies harsh reaction conditions, long reaction time, and generates a mixture of different shaped particles. There are various methods like hydrothermal process, 11 thermal decoma Electrochemical Materials Science (ECMS) Division, Central Electrochemical Research Institute (CECRI), CSIR-CECRI, Karaikudi , Tamil Nadu, India. skundu@cecri.res.in; subrata_kundu2004@yahoo.co.in; Fax: ; Tel: b Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai , India 482 CrystEngComm, 2013, 15, This journal is ß The Royal Society of Chemistry 2013

2 position, 12 sol gel, 13 electrochemical deposition, 14 sputtering, 15 spray pyrolysis 16 etc. that have been used to fabricate CoO nanostructures. Yin and Wang reported the synthesis of CoO nanocrystals with tetrahedral shapes by oxidation of Co 2 (CO) 8 in toluene at a temperature of 130 uc. 17 Zhang et al. obtained different CoO nanocrystals via decomposition of Co(II) oleate complex at uc. 18 Seshadri et al. reported the synthesis of spherical CoO NPs by decomposition of Co(acac) Hyeon and co-workers synthesized pencil shaped CoO nanostructures by the decomposition of Co(II) oleate complex at higher temperature. 20 Do and Wang prepared CoO particles from the calcination of Co(OH) 2 under N 2 atmosphere. 21 Zhan et al. synthesized CoO fibers of diameter y1 3 mm and several hundred millimetres in length by a pyrolytic process. 22 Recently, Ghosh et al. synthesized CoO NPs of diameter y nm range by decomposition of Co(II) cupferronate in decalin at 270 uc under solvothermal conditions. 23 However, most of the above processes take a longer time, need high reaction temperature and produce mixed phases of different cobalt oxides with a mixture of different shapes having non-uniform particle size distribution. Recently, the microwave (MW) heating method has been used widely for the synthesis of nanomaterials with significantly higher speed than conventional methods. The MW heating method increases the reaction rate significantly due to its high penetration as well as concentrated power. Moreover, the main advantages of MW heating are that it can heat a substance uniformly and generate more nucleation sites compared with conventional methods. In addition, it produces a narrow size-distribution and highly pure materials. There are a few reports for the synthesis of oxide based nanomaterials using the MW heating method Polshettiwar et al. synthesized several metal oxide nanomaterials using the MW heating method but the process generates mostly the larger particle size distribution. 24 Recently, Bhavikatti et al. synthesized oxide ceramics by MW heating methods. 25 Kundu et al. synthesized shape-controlled Au and Ag NPs using MW heating method in a short reaction time in presence of a surfactant or a polymer precursor. 28,29 Very recently, Bhatt et al. synthesized spinel Co 3 O 4 nanostructures by reduction of cobalt salt in presence of ethylene glycol and trioctylphosphine oxide (TOPO). 27 There are a couple of other reports for the formation of oxide nanostructures using the MW heating method. 30,31 Most of the previous methods of making CoO nanostructures require addition of seed particles, multiple steps, long reaction time, or generate a mixture of different shaped particles with lower yields. Nowadays, nanomaterials have been used extensively as catalysts for the removal of harmful organic compounds, polluted air and in wastewater treatment. 32 Different dye molecules are found to be toxic, nonbiodegradable, resistant to direct degradation by sunlight, and appear as a class of persistent pollutants 33,34 and become risky for the environment. Activated carbon assisted photocatalytic treatment has been used a lot for the removal of dyes prior to releasing the treated material into water due to their high surface area. 35,36 Alternatively, nanomaterials can help the degradation of the dyes by acting as a catalyst and help to minimize the number of steps required in textile effluent treatment. In this respect, CoO nanowires have been recognized as highly nontoxic, inexpensive materials for the catalytic mineralization of dyes to use for clean technology. In this present research, we developed a process using microwave heating to synthesize different sizes of CoO nanowires. The reduction of Co(II) ions was done by CTAB micellar media in presence of a new reducing agent alkaline 2,7-dihydroxy naphthalene (2,7-DHN). The reaction mixture was irradiated using MW for a total time of 6 min. The process exclusively generates CoO nanowires of different lengths just by tuning the metal-ion-to-surfactant molar ratios and changing the other reaction parameters. The synthesized CoO nanowires are found to serve as an effective catalyst for the reduction of several organic dye molecules in the presence of NaBH 4 in a short reaction time and assist the room temperature mineralization by providing a cleanup measure of a dye contaminated water body even in the presence or in the absence of light. To the best of our knowledge, synthesis of CoO nanowires of different lengths within 6 min of MW heating and their catalytic activity has not been previously reported. The yield of the CoO nanowires with uniform shapes are found to be significantly high (.95%) and the solutions with nanowires are stable for more than a month under ambient conditions. Moreover, the synthesis method and the catalysis process are very simple, reproducible, cost-effective and robust. Experimental section Paper Reagents The hydrated cobalt(ii) chloride hexahydrate (CoCl 2?6H 2 O) and sodium hydroxide (NaOH) were obtained from Sigma-Aldrich and used as received. The 2,7-dihydroxynaphthalene (2,7- DHN) was purchased from Sigma-Aldrich. Cetyltrimethylammonium bromide (CTAB, 99%) was purchased from Sigma-Aldrich and used as received. Three different dye molecules, Rose Bengal (C 20 H 2 C l4 I 4 Na 2 O 5, RB), methylene blue (C 16 H 18 N 3 SCl, MB), and Rhodamine B (C 28 H 31 N 2 O 3 Cl, RhB) (Sigma-Aldrich) were used as received. Sodium borohydride (NaBH 4 ) (Sigma-Aldrich) was prepared just before the reduction reaction and used fresh daily and always stored inside the refrigerator. De-ionized (DI) water was used for the entire synthesis and catalysis work. Instruments The synthesized CoO nanowires were characterized with several spectroscopic techniques. The UV-visible (UV-vis) absorption spectra were recorded in a Hitachi (model U-4100) UV-vis-NIR spectrophotometer equipped with a 1 cm quartz cuvette holder for liquid samples. The high resolution transmission electron microscopy (HR-TEM) analysis was done with a Tecnai model TEM instrument (Tecnai TM G2 F20, FEI) with an accelerating voltage of 200 kv. The scanning electron This journal is ß The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15,

3 Paper microscopy (SEM) analysis was done with HITACHI Model S-3000H instruments. The energy dispersive X-ray spectroscopy (EDS) analysis was done with the SEM instrument with a separate EDS detector connected to that instrument. Atomic force microscopy (AFM) images were recorded with a Picoscan 2000 model and operate at a resonant frequency of khz. The instrument can measure both contact and lateral force having sensitivity of 10 na and resolution of 0.3 mv. The X-ray diffraction (XRD) analysis was done using a PAN analytical Advanced Bragg Brentano X-ray powder diffractometer (XRD) with Cu Ka radiation (l = nm) with a scanning rate of s 21 in the 2h range 10 90u. The X-ray photoelectron spectroscopy (XPS) analysis was carried out using a ESCA model VG 3000 system X-ray photoelectron spectrometer with monochromatic Mg Ka line ( ev) radiation. The instrument integrates a magnetic immersion lens and charge neutralization system with a spherical mirror analyzer, which provides real time chemical state and elemental imaging using a full range of pass energies. The emitted photoelectrons were detected by the analyzer at a passing energy of 20 ev with energy resolution of 0.1 ev. The incident X-ray beam was normal to the sample surface, and the detector was 45u away from the incident direction. The analysis spot on the sample was 0.4 mm mm. The overall energy resolution was about 0.8 ev. Samples for the survey spectrum was recorded in the ev kinetic energy by 1 ev steps whereas high resolution scan with 0.1 ev steps were conducted over the following regions of interest: Co 2p, O 1s, C 1s and Br 3d. The Fourier transform infrared (FT-IR) spectroscopy analysis was done with the model Nexus 670 (FT-IR), Centaurms 106 (microscope) having spectral range 4000 to 375 cm 21 with a MCT-B detector. The dc magnetization measurements were carried out as a function of temperature and magnetic field using a Physical Properties Measurement System (Cryogenic, UK make). A domestic microwave (MW) oven (Samsung Company, DE B) was used for MW irradiation for the entire synthesis. The output power was W and the operating frequency was 2450 MHz. Microwave synthesis of CoO nanowires CoO nanowires with different lengths were synthesized by tuning the concentration of CTAB and Co(II) ions in the reaction mixture containing CoCl 2?6H 2 O, 2,7-DHN, CTAB and NaOH. For a typical synthesis process, 50 ml of CTAB (0.1 M) was mixed with 10 ml of (10 22 M) CoCl 2?6H 2 O solution. To CrystEngComm this 10 ml of 2,7-DHN (10 22 M) solution and 1 ml of NaOH (1 M) solution were added. The solution mixture was stirred for 20 s using a magnetic stirrer, which was then irradiated by MW for about 6 min with an intermittent pause after every 10 s to cool the reaction vessel. The process exclusively generates CoO nanowires with shorter length. For the synthesis of nanowires with various length we varied the concentrations of CTAB and Co(II) ions keeping the time of MW irradiation fixed in all cases. The final concentration of all the chemicals and other reaction parameters are given in detail in Table 1. After the addition of NaOH and stirring for 20 s, the solution was light bluish. During MW irradiation for 1 min, the solution showed a bluish green colour, after 2 min it became deep blue, after 4 min it changed to bluish black and finally after 6 min it turned blackish in colour. The solution mixture was centrifuged at 6000 rpm for 20 min and again at 4000 rpm for 15 min to remove excess CTAB and other unwanted chemicals from the NPs solution. The precipitated brownish color CoO nanowires solutions were re-dispersed in DI water and stored in a refrigerator covered in black paper. The yield of the CoO nanowires with uniform shapes are found to be significantly high (.95%) and the nanowires solutions are stable for more than a month without changing any optical properties. Catalytic mineralization of organic dye molecules using CoO nanowires as catalyst The mineralization of three different organic dye molecules like RB, MB, and RhB in presence of NaBH 4 using CoO nanowires as catalyst was studied in detail. A stock solution of three different dyes (10 23 M) was prepared and used for the entire reduction study. A fresh stock solution of NaBH 4 (0.1 M) was prepared freshly daily and was stored in a refrigerator in the dark. For the mineralization study, the dye solution was diluted to a concentration of M. The whole reaction was carried out using a quartz cuvette having a path length of 1 cm. For a typical mineralization study, 8 ml of (10 25 M) dye solution was mixed with 1.75 ml of 0.1 M NaBH 4 solution and the solution was mixed well by vigorous shaking. Finally, 250 ml of CoO nanowires (shorter length) was added, mixed well, and the progress of the reduction was monitored spectrophotometrically using an in-situ UV-vis spectrophotometer. The reduction of all the dyes started within a couple of minute and were completed within 130 min, as observed from the UVvis spectrum. After the completion of the reaction, the pinkish colored RhB, RB and bluish colored MB became colorless, and Table 1 The detailed final concentrations of all the reaction parameters, time of MW irradiation, and particle size and shape distribution for the formation of CoO nanowires Set no. Final conc. of CTAB (M) Final conc. of Co(II) ions (M) Final conc. of 2,7-DHN (M) Final conc. of NaOH (M) Time of MW heating (min) Shape of the CoO NPs Lengths and diameter of the CoO nanowires Shape distribution Wires (short length) Wires (medium length) Wires (long length) L y nm, D y 5 2nm L y nm, D y 10 3nm L y nm, D y 15 2nm.95% wires.95% wires.95% wires 484 CrystEngComm, 2013, 15, This journal is ß The Royal Society of Chemistry 2013

4 Paper was further confirmed from the UV-vis spectrum. After the complete reduction of the dyes, the catalysis reaction rate was calculated and a comparison study was made as discussed in detail later in the results and discussion section. Preparation of samples for different instrumental (UV-vis, TEM, EDS, XRD, XPS, AFM and FT-IR) analysis The CoO nanowires were characterized by using UV-vis, TEM, EDS, XRD, XPS, AFM, and FT-IR measurements. The CoO nanowire solution after successive centrifugation was redispersed in DI water and used for the measurement in UVvis spectrophotometer. The samples for TEM were prepared by placing a drop of the corresponding CoO nanowire solution onto a carbon coated Cu TEM grid followed by slow evaporation of solvent under ambient conditions. For EDS, XRD, XPS, and FT-IR analysis, glass slides were used as substrates for thin film preparation. The slides were cleaned thoroughly in acetone and sonicated for about 30 min. The cleaned substrates were found to be covered with the CoO nanowires solution and then dried in air. After the first layer was deposited, subsequent layers were deposited by repeatedly adding more CoO nanowires solution and drying. Final samples were obtained after 8 10 depositions and then analyzed using the above techniques. For SEM and AFM analysis, the samples were prepared in a similar way as discussed before on glass slides but only a single deposition was done. Results and discussion UV-vis spectroscopic analysis CoO nanowires of different lengths were prepared by the reduction of Co(II) ions in the presence of alkaline 2,7-DHN in CTAB micelle media under 6 min of MW irradiation. The hydrated Co(II) chloride in aqueous solution forms a pink color solution due to the formation of [Co(H 2 O) 6 ] 2+ complex. 37 Now in the presence of alkali, the pink color solution changed to light bluish color either due to change in co-ordination of metal ion or due to formation of [Co(H 2 O) 4 (OH) 2 ] complex. The overall reaction changes the color of the solution from pink to blue. This blue color solution contained a neutral complex, cobalt hydroxide, which is insoluble in water and has a strong tendency to precipitate. The equations for the above reactions are given below. Co 2+ +6H 2 O P [Co(H 2 O) 6 ] 2+ [pink color] [Co(H 2 O) 6 ] 2+ +OH 2 P [Co(H 2 O) 5 (OH)] + +H 2 O [Co(H 2 O) 5 (OH)] + +OH 2 A{Co(H 2 O) 4 (OH) 2 ]+H 2 O [blue color] Fig. 1 shows the UV-vis spectra of the reaction mixture at different stages of the synthesis process. An aqueous solution of CTAB has a small absorption band peaking near 300 nm as shown in curve A in Fig. 1. Light pink color aqueous Co(II) solution has a broad absorption band at nm (curve B, Fig. 1 The UV-vis spectrum of the reaction mixture at different stages of the synthesis process. (A) Absorption spectra of an aqueous solution of CTAB. (B) Absorption band of pink color aqueous Co(II) solution. (C) Absorption band of a mixture of aqueous CTAB, Co(II), and 2,7-DHN solution. (D) Absorption band of a mixture of aqueous CTAB, Co(II), 2,7-DHN and NaOH solution. (E), (F) and (G) shows the absorption bands of short, medium and large lengths CoO nanowires respectively. The inset shows the camera images of three different CoO nanowires indicated as E1 (short lengths), F1 (medium lengths), and G1 (long lengths). Fig. 1) due to a ligand to metal charge transfer (LMCT) process. 38 An aqueous colorless solution of 2,7-DHN shows two distinct absorption bands peaking at 280 nm and 323 nm due to the presence of aromatic rings (not shown here). A mixture of aqueous CTAB, Co(II), and 2,7-DHN shows three small absorption bands peaking at 285 nm, 314 nm and 329 nm (curve C, Fig. 1) probably due to the formation of complex of CTAB with Co(II) or 2,7-DHN. Now after addition of NaOH to the reaction mixture containing CTAB, 2,7-DHN and Co(II), intensity of the previous peaks (curve C, Fig. 1) reduced down and a new peak at 343 nm (curve D, Fig. 1) appeared probably due to the formation of blue complex as shown in the equation above. Once the MW heating started, the solution color changed step by step with time as shown in Scheme 1. After 2 min of MW heating, the solution color was deep blue, after 4 min it showed bluish black color and after 6 min it became blackish in color. The solution was centrifuged (details given in the experimental section) and contained exclusively CoO nanowires. The UV-vis spectra of short length nanowires are shown in curve E, which contains two absorption bands peaking at 269 nm and 405 nm, respectively. Similarly, medium length nanowires and long length nanowires also showed similar types of absorption behavior with a slight change in absorption spectra as shown in curve F and curve G, respectively. Curve F shows two bands at 271 nm and 410 nm for medium length nanowires whereas curve G shows two bands at 279 nm and 418 nm for long length nanowires. These absorption bands have similarity with the results reported by others for the synthesis of cobalt oxide nanomaterials Apart from these absorption bands for CoO nanowires (in curve E, F and G in Fig. 1) there is also a small hump observed near 300 nm that is probably due to the presence of a little This journal is ß The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15,

5 Paper CrystEngComm Scheme 1 The successive change in color with time during the formation of CoO nanowires using MW heating. excess of surfactant or other reaction products in the CoO nanowire solution mixture that we are unable to remove by repeated centrifugation. However, these absorption bands for CoO nanowires can be shifted with the alteration of solvent due to a change in refractive index of the medium. The synthesized CoO nanowires are stable for more than a month and stored in a sealed bottle in a refrigerator at 4uC for long term use. The inset of Fig. 1 shows the pictures of the solutions containing three different length CoO nanowires indicated with E1 (short length nanowires), F1 (medium length nanowires), and G1 (long length nanowires) after successive centrifugation and re-dispersion in DI water. Transmission electron microscopy (TEM) analysis Fig. 2 shows the transmission electron (TEM) images of CoO nanowires with different lengths at various magnifications after 6 min of MW irradiations. Fig. 2A and 2B show the low and high magnified TEM images of short length nanowires. From Fig. 2A, the average length (L) of the wires is y50 10 nm and diameter (D) is y5 2 nm. Fig. 2B shows the corresponding high magnified image and the inset of Fig. 2B shows the selected area electron diffraction (SAED) pattern confirming that the particles are nanocrystalline. Fig. 2C and 2D show the TEM images of medium length CoO nanowires at different magnifications. From Fig. 2C, the average length of the wires is y nm and diameter is y10 3 nm. Fig. 2D shows the corresponding high magnified images and the inset shows the SAED pattern, which confirms the nanocrystalline nature of the synthesized materials. Fig. 2E and 2F show the TEM images of long length CoO nanowires at different magnifications. Fig. 2E shows the low magnified image having the average length of the wires y nm and the diameter y15 2 nm. Fig. 2F shows the high magnified image of a single nanowire where we can see the orientation of different crystal planes and the distance between two individual crystal planes is y0.363 nm. The inset of Fig. 2F shows the corresponding SAED pattern that confirms the presence of crystallites in the material. From the above TEM analysis, it is confirmed that we are able to synthesize uniform CoO nanowires of various lengths by changing the reaction parameters and in all the cases the nanowires are showing crystalline behavior. Moreover, one interesting thing to be noted is that with the decrease in CTAB concentration and increase in other reagents concentrations, the length of the CoO nanowires as well as their diameter increases in our proposed synthesis route. From the TEM analysis we observed that.95% CoO nanowires are formed with uniform shapes. Apart from these nanowires we also observe some black parts in the TEM pictures that are due to the presence of small amount of excess surfactant or other reaction products that we are unable to remove by repeated centrifugation. There are no cobalt or oxygen peaks observed from those dark patches in the EDS analysis, clearly confirming that those are not the CoO particles. Energy dispersive X-ray spectroscopy (EDS) analysis Fig. 3 depicts the results obtained from energy dispersive X-ray spectroscopy (EDS) analysis. An EDS analysis is generally used to determine the elements present in the reaction product. The EDS spectrum consists of different peaks corresponding to Co, O, Si, and Br. The large, intense Co and O peaks came from the CoO nanowire samples. The Si peak came from the glass substrate used for making the sample for EDS analysis. A small intense Br peak also appears due to presence of CTAB used in our synthesis process, which acts as a capping agent for the synthesis of CoO nanowires. X-ray diffraction (XRD) analysis The X-ray diffraction (XRD) pattern of CoO nanowires are recorded (Fig. 4) and the peaks observed in the range 10 90u confirm the formation of nano phase CoO having FCC 486 CrystEngComm, 2013, 15, This journal is ß The Royal Society of Chemistry 2013

6 Paper Fig. 2 The transmission electron microscopy (TEM) images of shape-selective CoO nanowires. (A) and (B) show the low and high magnified TEM images of short lengths nanowires and the inset of (B) shows the selected area electron diffraction (SAED) pattern. (C) and (D) show the TEM images of medium lengths CoO nanowires and the inset shows the SAED pattern. (E) and (F) show the low and high magnified images of the large lengths nanowires and the inset of (F) shows the corresponding SAED pattern. structure (JCPDS card number ; space group Fm3m) with no impurity detected. 42,43 The peaks at 2h values 20.3, 39.1, 50.7, 62.2, 65.5, and 69.2 are assigned to scattering from the (111), (222), (422), (511), (220) and (440) planes, respectively. The size of the CoO nanomaterials was measured from the X-ray diffraction peak line width broadening using the Debye Scherrer formula for small crystalline spheres. The mean diameter of the particles are consistent with the result obtained from TEM analysis at Fig. 2 which is in between y nm range for the different types of nanowires. The sharp diffraction peaks indicate that the presence of CoO particles with good crystallinity and the broad diffraction peaks indicate the nano size nature of the particles. All the diffraction peaks are nicely matched with the previous results reported by Hyeon et al. 20 and Lagunas et al. 43 In our synthesis we used CTAB as a capping agent and the selective interaction of CTAB with different crystal planes of CoO might have changed the growth rates of the different crystal planes and also their XRD peak intensities. X-ray photoelectron spectroscopic (XPS) study X-ray photoelectron spectroscopy (XPS) analysis was used to investigate the elements present at the surface as well as the oxidation state of the metal atom at the materials surface. An Al Ka XPS survey scan was conducted for the thin films deposited onto glass substrates, which showed the presence of the Co 2p, Co 1s, O 1s, C 1s and Br 3d peaks at their respective binding energy regions. Fig. 5 shows the XPS spectrum of CoO This journal is ß The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15,

7 Paper CrystEngComm lower binding energies, ev for O 1s (Fig. 5C) and ev for C 1s (Fig. 5D), were observed. The small intense Br (3d) peak was observed at a binding energy of 61.1 ev (Fig. 5E) and that came from the surfactant CTAB used as a capping agent to stabilize the CoO nanowires. We have not observed any peak for Si due to the large thickness of the deposited materials although we used glass as substrate for making the samples. Fig. 3 The energy dispersive X-ray spectroscopic (EDS) analysis of CoO nanowires. nanowires. Fig. 5A shows the overall survey spectrum, which consists of the characteristic peaks of C 1s at ev, O 1s at ev, Co 3s at ev, Co 2s at ev, Br 3d at 61.1 ev. The Co 2p regions are characterized by a doublet that arises due to spin orbit coupling (2p 3/2 and 2p 1/2 ). The peak positions are located at and ev for Co 2p 3/2 and Co 2p 1/2, respectively, as shown in Fig. 5B. These two binding energies of Co 2p 3/2 and Co 2p 1/2 in the XPS spectra of CTAB coated CoO nanowires correspond to cobalt in +2 oxidation state. The shake-up satellite peaks for Co 2p 3/2 and Co 2p 1/2 were observed at 789 ev and 804 ev, respectively (Fig. 5B). The presence of two peaks at and ev (main peaks) and an intense satellite peak is consistent with the presence of Co 2+ in the high spin (4 F ) state. The absence of any type of feature at ev indicates the nonexistence of Co metal impurity. All the above peaks are well matched with the previous reports for CoO nanomaterials. 23,42 44 Other peaks at Fig. 4 The powder X-ray diffraction (XRD) pattern of CoO nanowires. Fourier transforms infrared (FT-IR) spectroscopic analysis Fig. 6 shows the Fourier transform infrared (FT-IR) spectra of the pure 2,7-DHN, pure CTAB and CTAB bound CoO nanowires. Fig. 6A shows the FT-IR spectra of pure 2,7-DHN that contains all the expected peaks and clearly matches with the literature. 45 A comparison of the FT-IR spectra of pure CTAB (Fig. 6B) with the dried samples of washed CTAB coated CoO (Fig. 6C) nanowires not only supports the presence of CTAB but also reveals the nature of interaction of CTAB molecules with CoO nanowires. It is well known that FT-IR spectra of CTAB capped metal or metal oxide NPs strongly depends on particles size. The normal N H stretching frequencies of CTAB comes around 3450 cm 21 but in the case of CTAB bound CoO it is blue shifted and comes around 3421 cm 21. It proves that CTAB is bound with the CoO nanowires surface. So it seems that CTAB might bind with its head group to the CoO NPs surface and give rise to an unfavorable interaction by putting the hydrophobic tails of the surfactant towards the aqueous environment and another layer of CTAB head group bound towards the aqueous environment forms a double layer as reported by others. The two characteristic FT- IR peaks appearing at 493 and 660 cm 21 correspond to the metal oxygen (n Co O ) stretching vibration modes of CoO. In general, metal oxides give absorption bands below 1000 cm 21 arising from inter-atomic vibration. This confirms the formation of CoO. 27,46,47 We have not observed any peaks around 3630 cm 21, which clearly confirms the absence of any OH group or moisture on the CoO nanowire samples. Two small intense peaks appearing at 2834 cm 21 and 2895 cm 21 may be due to the presence of surfactant molecules and the C H stretching vibrations associated with them. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) studies Fig. 7 shows the scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of CoO nanowires. From the SEM images in Fig. 7A and 7B, rod like morphology of the CoO nanowires is clearly observed. The image is not very clear due to the limitation of the SEM instrument, as we are unable to reach much higher magnification. Fig. 7C and 7D show the tapping mode AFM images of CoO nanowires for 2D topography and 3D topography, respectively, using silicon nitride cantilever. From the image it seems that bunches of nanowires are aggregated together although we observed individual CoO nanowires from TEM analysis. From the UVvis absorption study also we got the absorption peaks related to the dispersed nanowires but not for the aggregated nanoparticles. So we assume that the individual CoO nanowires are agglomerated together during sample preparation and generate the types of topography seen in SEM or in AFM analysis. The average diameter and length of the 488 CrystEngComm, 2013, 15, This journal is ß The Royal Society of Chemistry 2013

8 Paper Fig. 5 The X-ray photoelectron spectroscopy (XPS) of CoO nanowires. (A) Shows the overall survey; (B) spectra of Co 2p; (C) spectra of O 1s; (D) spectra of C 1s; (E) spectra of Br 3d. This journal is ß The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15,

9 Paper CrystEngComm 2,7-DHN and observed that at low concentration of 2,7-DHN, longer MW irradiation time is needed for the formation of particles and at a very high concentration of 2,7-DHN, the particles are precipitated from the solution. Similarly, at a reasonable concentration of NaOH the solution produced specified shapes but at a very high or low NaOH concentration it either produced CoO NPs with undefined shapes or the reaction takes a very long time to generate the nanomaterials. Further, in most of the cases the generated particles are not stable. In addition to the above controlled experiments we also checked the effect of MW irradiation time. We have seen that 6 min irradiation time is sufficient to produce CoO nanowires with different lengths, as given in Table 1. So in conclusion, all the above controlled experiments prove that the selection of proper concentrations of reagents is extremely important for the formation of CoO nanomaterials with uniform shape and defined morphology. Fig. 6 The Fourier transform infra red (FT-IR) spectra of the pure 2,7-DHN, pure CTAB and CTAB bound CoO nanowires: (A) shows the FT-IR spectra of pure 2,7- DHN; (B) shows the FT-IR spectra of pure CTAB and (C) shows the FT-IR spectra of CTAB coated CoO nanowires. nanowires are calculated from the SEM and AFM images, which nicely matched with the results obtained from TEM analysis. From both the SEM and AFM analysis we can see that the image is not perfectly clear due to instrument limitations as well as due to the presence of a small amount of excess surfactant in the reaction mixture as we discussed earlier. Study of other reaction parameters To standardize the synthesis process and to get uniform CoO nanowires we have conducted some controlled experiments to check the effects of other reaction parameters. We checked our synthesis process with different concentrations of CTAB, Co(II) ions, 2,7-DHN, NaOH, and also by changing the MW heating time. The shape controlled CoO nanowires of different lengths are formed at particular concentrations that are given in detail in Table 1. From Table 1, it is clearly observed that shape selective CoO nanowires of different lengths are formed with variation in concentrations of CTAB and Co(II) ions. We can see that at higher CTAB concentration (y10 21 M) mostly short nanowires are formed. At a medium CTAB concentration (y10 22 M) mostly intermediate size nanowires are formed. When the CTAB concentration is very low ( or M), mostly spherical NPs with a mixture of a few other anisotropic shapes are formed. In another case, when CTAB concentration is too high ( M) the reaction takes a longer time and does not produce the required materials in a reasonable time scale. We have seen that when the Co(II) ion concentration is very low ( M), a longer MW irradiation time is needed to initiate and complete the formation of CoO particles whereas when the Co(II) ion concentration is very high ( M), CoO particles are formed but with no specific shapes. Similarly, we have tested the concentration of Mechanisms of CoO nanowires formation The CoO nanowires with different lengths are synthesized by the reduction of Co(II) ions in CTAB micellar medium in the presence of alkaline 2,7-DHN under MW irradiation. The overall reaction was carried out at room temperature and under ambient conditions. In our proposed reaction, the presence of CTAB and alkaline 2,7-DHN is extremely important for the formation of defined shaped particles. Keeping other reagents the same but in the absence of CTAB, CoO particles are formed but are immediately precipitated due to the absence of any stabilizer. In the absence of 2,7-DHN, no nanomaterials are formed due to the lack of a specific reducing agent in the reaction mixture. A specific concentration of NaOH also played an important role for the formation of CoO nanowires. In our study the ph of the solution mixture was in the range of 8 9.5, which can promote the reducing power of 2,7-DHN and initiate the growth of NPs with defined shapes. It was reported previously that only a specific concentration of OH ions can direct the growth of NPs into various anisotropic shapes. 48 Moreover, Murphy et al. reported the importance of OH 2 ions for the formation of anisotropic particles. 49 It was observed earlier that several hydroxylic compounds like ascorbic acid, 50 TX-100, 51 poly(vinyl)alcohol, 52 dendrimer 53 etc. having a hydroxylic group on their structure can generate phenoxy radicals in the presence of UV irradiation, MW heating or heating at very high temperature. This radical species generated can act as a reducing agent for the reduction of metal ions to metal 0. Kundu et al. reported earlier that hydroxylic compounds can reduce Au(III) and Ag(I) to generate the corresponding zero valent state with a different morphology. 28,51 It was also observed that poly(vinyl)pyrrolidone (PVP) having an amine group can also act as a reducing agent in the presence of MW heating for the formation of Au NPs. 54 So in our proposed process we also assumed that the radical species generated can promote the reduction of cobalt ions to Co(0). Once the Co(0) is formed, it reacts with oxygen to produce CoO nanomaterials. At the early stage small Co nuclei grow to form Co atoms and then crystalline Co particles. Those Co particles are not stable in ambient conditions and readily react with oxygen and produce CoO. The surfactant CTAB molecules adsorb onto the surface of the particles and slow 490 CrystEngComm, 2013, 15, This journal is ß The Royal Society of Chemistry 2013

10 Paper Fig. 7 Scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of CoO nanowires. (A) and (B) show the SEM images at different magnification; (C) and (D) show the tapping mode AFM image for 2D topography and 3D topography of CoO nanowires, respectively. down the growth rate of different crystal facets. As we discussed earlier when CTAB concentration is low (,10 25 M), the interaction of cationic part of the surfactant with different crystal facets is weak and growth takes place in all possible directions and generates mostly spherical CoO NPs. It was extensively studied by Murphy 55 and Shellnutt s 56 groups that a higher concentration of CTAB (y M) can generate polygonal, rod shaped or worm-shaped micellar template leading to the change in morphology of the metal. In our proposed study, we observed that CoO nanowires are formed at a higher concentration of surfactant, which matches with the previous reports. So, we also believed that at high CTAB concentration the surfactant can form a rod/worm shaped micellar template and once particles grow on that This journal is ß The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15,

11 Paper CrystEngComm Scheme 2 Schematic presentation for the formation of CoO nanowires with different sizes. template, they generate the nanowires. In the given concentration discussed in Table 1, we observed.95% CoO nanowires with uniform shapes, although there are very small amounts of excess surfactant that we are unable to remove by repeated centrifugation that can also be observed from the TEM and SEM analysis. It was previously reported that the growth of any metallic NPs mainly depends upon two things. 57 The first is the faceting tendency of surfactant and the second is the growth kinetics, which is the rate of supply of metal(0) to the different crystallographic planes. In our study the formation of different sized nanowires are schematically shown in Scheme 2. From Scheme 2 we can see that at a lower CTAB concentration large size nanowires are formed, whereas at a higher CTAB concentration short size nanowires are grown. From the TEM images we can observe that the particles are mostly separated from each other due to the presence of the surfactant, which can stabilize the particles as a protective shell from unwanted aggregation. However, in few cases, there is some free surfactant (very little) that we are unable to remove by repeated centrifugation and is observed both in TEM and SEM analyses. After the synthesis of CoO nanowires of various lengths we have studied the catalytic activity for the mineralization of some organic dye molecules using NaBH 4 as a reducing agent. For an example we used smaller size CoO nanowires as catalyst for the detailed mineralization study. Magnetic studies of CoO nanowires The cobalt oxide crystallizes normally in rock salt structure which is antiferromagnetic (T N y298 K) and electrically insulating. 58 We already discussed in the introduction section that it is not easy to synthesize CoO in bulk form as in most cases it is contaminated with Co 3 O 4 or Co metal. Moreover, when the size of the particles becomes smaller, thermal energy will compete with the magnetic anisotropy energy. This tends to decrease the Curie temperature. 59 Bulk CoO shows an antiferromagnetic transition at the Neel temperature of 293 K. 60 In our study we did the magnetic measurement taking different sizes of CoO nanowires for comparison. Fig. 8 depicts the temperature and field dependence of magnetic moment for CoO nanowires with three different lengths. As evident from the magnetic moment (M) vs. T curves (Fig. 8A) for the short and medium length CoO nanowires, there has been a sharp rise in magnetic moment below y19 K, whereas for the longer nanowires, no sharp rise in magnetic moment has been observed. The medium and long length CoO nanowires show a hysteretic behavior at 5 K, but no saturation of magnetization is observed for any of these samples. No observable coercivity 492 CrystEngComm, 2013, 15, This journal is ß The Royal Society of Chemistry 2013

12 Fig. 8 The magnetic measurement curves for the CoO nanowires with three different lengths. (A) The magnetic moment (emu) vs. temperature (K) curves and (B) the magnetic moment (emu) vs. magnetic field (T) curves for different length nanowires. has been recorded for the short length nanowires as seen in Fig. 8B, whereas, the coercivities of y0.01 and 0.06 T have been observed for the medium and long length nanowires, respectively. As the lengths of the nanowire have been increased, keeping the length to diameter ratio almost constant, an increase in the coercivity has been observed. However, the magnetic moment values at the maximum applied magnetic field of 2 T have been decreased with increase in the lengths of the nanowires. Catalytic mineralization of organic dye molecules using CoO nanowires as catalyst The ability of CoO nanowires as catalyst was tested in detail for the mineralization of organic dye molecules using NaBH 4 as reducing agent. For the catalysis study, the synthesized CoO nanowires are centrifuged several times to remove and minimize the excess CTAB in the nanowire solution. After repeated centrifugation we redispersed the CoO nanowires in DI water and they used fresh for the catalysis study. It is well known that for any type of catalysis reaction, either homogeneous or heterogeneous, the reducing agents need a fresh catalyst surface for fast electron transfer. In case there is some excess polymer or surfactant trapped on the particles surface, the catalysis rate will be slow. For this reason we need to Paper remove the excess surfactant for better catalytic efficiency. Here in this catalysis study, we selected three different types of organic dye molecules, two anionic dyes, RB and RhB, and one cationic dye, MB, having different organic skeletons. All of the three dye molecules are highly soluble in water and used as commercial colorants in the dying industry. It is accepted that most of the unused dye molecules mix with water and create problems for environmental pollution. There are couple of methods reported earlier for the degradation of dyes, but most of them require harsh reduction conditions, multiple steps and not seem to be environmentally friendly The dye reduction was studied earlier using UV light but in most of the cases that is not good due to health concerns. 64 The synthesized CoO nanowires assist the room temperature mineralization of the different dye molecules and provide a cleanup measure of a dye contaminated water body even in the presence or in the absence of light. Here all the three dye molecules were reduced in the presence of NaBH 4 and CoO nanowires. CoO nanowires act as a catalyst for this reduction. As a representative study we used short length CoO nanowires as catalyst. The overall reaction can be studied at room temperature and nicely monitored using a UV-vis spectrophotometer. The ph values of RB, RhB and MB (y10 25 M) solutions in water are 6.88, 6.78 and 7.52, respectively. The ph of M NaBH 4 is The reduction of these dyes without catalyst but in the presence of NaBH 4 is very slow. Similarly, only by using CoO nanowires, in the absence of NaBH 4, reduction of these dyes does not take place at all even after 7 days. So a proper combination of all the three reagents (dye molecule, NaBH 4 and CoO nanowires) is extremely important for the fast catalysis reaction. The UV-vis spectra of RB, RhB, and MB show an intense absorption maximum at 549 nm, 552 nm and 663 nm, respectively. After the addition of NaBH 4 and CoO nanowires with the dye solution, the reaction starts spontaneously or within a couple of minutes depending on the type of dye molecules used. The successive reduction of the absorbance value can be nicely monitored using a UV-vis spectrophotometer. Fig. 9A shows the successive decrease of the absorbance value for RB and the reduction is completed after 130 min. Fig. 9B shows the ln(abs) vs. T (time) plot for the RB reduction and the rate constant value calculated from the first order rate equation is min 21. Similarly, Fig. 9C and 9D show the successive reduction of the absorption maxima for MB and the corresponding ln(abs) vs. T (time) plot respectively. The reduction is completed after 81 min and the first order rate constant value is min 21. Similarly, Fig. 9E and 9F show the UV-vis spectra for the reduction of RhB and the corresponding ln (abs) vs. T (time) plot respectively. The rate constant value for RhB reduction is min 21. The overall concentration of dye molecules, reduction times, and rate constant values are summarized in Table 2. From Table 2 we can clearly see that the reaction is fastest in case of RhB, intermediate with MB and slowest with RB. In all cases, BH 4 2 transfers electrons to the dye molecule and the dye molecule gets reduced. This electron transfer took place via the CoO nanowires to the dye molecules and the dyes become colorless after reduction. The insets of Fig. 9A, 9C and 9E show the color of the dyes before reduction and insets of Fig. 9B, 9D and 9F show the colorless This journal is ß The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15,

13 Paper CrystEngComm Fig. 9 The UV-vis absorption spectrum for the mineralization of three different dye molecules with NaBH 4 using short length CoO nanowires as catalyst. (A) is the UVvis spectrum for the successive reduction of RB and (B) is the ln(abs) vs. time (T) plot having the rate constant value min 21. (C) and (D) are the UV-vis spectrum for the successive reduction of MB and ln(abs) vs. time (T) plot, respectively, having the rate constant value min 21. Similarly, (E) and (F) are the successive reduction for RhB and ln(abs) vs. time (T) plot curves, respectively, having rate constant value min 21. The insets of (A), (C) and (E) show the color of the dyes before reduction and insets of (B), (D) and (F) show the colorless dyes after reduction. dyes after the reduction. The overall dye reduction process is schematically shown in Scheme 3. From Table 2 and Scheme 3, we can clearly observe that the dye reduction is fastest with RhB and slowest with RB, probably due to better adsorption of RhB onto the catalyst surface compared with MB. At this point it is not fully clear why the reaction rate is 494 CrystEngComm, 2013, 15, This journal is ß The Royal Society of Chemistry 2013

14 Paper Table 2 The final concentration of all the reactants, mineralization time, rate constant values for the mineralization of dye molecules with NaBH 4 in presence of CoO nanowires as catalyst Name of the dye molecules Final conc. of dye (M) Final conc. of NaBH 4 (M) Volume of CoO nanowires (ml) Time for full mineralization (min) First order rate constant, K (min 21 ) Correlation coefficient, R Standard deviation, SD Rose Bengal (RB) Methylene blue (MB) Rhodamine B (RhB) different for different dyes and further study is warranted to get a clear insight for this dye reduction process. Moreover, how the catalysis rate changes with the change in nanowire length will also be discussed in the near future. So the above catalysis process might be useful not only for small scale but also in large scale like industrial view points for the mineralization of variety of unused pollutants from our environment. Conclusion In summary, the shape-selective CoO nanowires have been successfully synthesized by a quick MW assisted synthesis route. The CoO nanowires were synthesized by the reduction of Co(II) ions using alkaline 2,7-DHN in the presence of CTAB surfactant media under 6 min of MW heating. The process exclusively generates CoO nanowires of different length and having diameter y5 2nmto15 2 nm range just by tuning the metal-ion-to-surfactant molar ratios and changing the other reaction parameters. For the short nanowires, there has been no observable coercivity, but for medium and long length nanowires, noticeable coercivities have been observed. As the length of the nanowires increases the coercivity increases, whereas the magnetic moment values decrease at the maximum applied magnetic field of 2 T. The synthesized CoO nanowires are stable for more than a month under ambient conditions and are found to act as efficient catalysts for the mineralization of several organic dye molecules in the Scheme 3 Schematic presentation for the overall dye mineralization process using CoO nanowires as catalyst. This journal is ß The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15,