A inorganic materials requires intricate technique such as

A facile surfactant free morphology controlled 3D flower-like synthesis of Nickel Tungsten Oxide microstructure




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·         Uniform 3D-NiWO4 were synthesized successfully by chemical reduction method.

·         The mechanism of 3D-NiWO4 flower in the aqueous solution is proposed.

·         Different parameter plays an important role to control the morphology.

·         3D-NiWO4 can be applied for non-enzymatic glucose sensing.




Three-dimensional (3D) microstructure of nickel tungsten oxide (NiWO4) flower-like controlled morphology were synthesized surfactant free through chemical reduction method at low temperature (60 C). The uniform morphologies of NiWO4 could be control by simply adjusting temperature, molar ratio, stirring rate and the dosage of reducing agent in aqueous solution. It is found that introducing of hydrazine monohydrate (N2H4.H2O) make the precipitation to prepare 3D-NiWO4 flower-like microstructure. The formation mechanism of self-assembly NiWO4 flower structure were characterized by X-ray di?raction (XRD), scanning electron microscopy (SEM), Transmission Electron microscopy (TEM), Energy dispersive X-ray spectroscopy (EDS) and Fourier Transmission Infrared Spectroscopy (FTIR).  Furthermore, 3D-NiWO4 flower-like microstructure were used for electrochemical properties using glassy carbon electrode.



3 D-NiWO4 flower; Characterization; Crystal structure; electrochemical properties



Graphical abstract




With the increasing demand, synthesis of controllable morphology inorganic materials has been predicted for material science research owing to their primal optical, electrical and catalytic properties. The morphology has outstanding performance in several applications including electronics, catalysis, bio-sensing and energy storage. In particular 3D architecture, metal oxide was synthesized through bottom-up approach such as flower-like ZnO, Iron oxide film, cobalt oxide microstructure, NiCoO nanoneedles, a-MnMoO4 hierarchical architectures. Based on morphology, metal oxide have dramatic output in the field of application such as controlled morphology flower-like NiCo2O4 delivered higher specific capacitance compare to nanowires.   Therefore, the focus on the material science research has inclined to engineering controllable morphology in order to boost efficiency in application field. However, the controlling morphology of inorganic materials requires intricate technique such as templet assisted, hydrothermal and sol-gel process. By observing high synthesis temperature and introducing surfactants amount as well as long process material preparation, the crystallinity and grain sizes may hinder while hydrothermal or sol-gel methods. Nevertheless, research of amiable and facile synthesis method for the construction of inorganic 3D flower-like inextricable structures without any surfactant and demanding construction along with mechanisms remain challenges in material science.


Recently, transition metal tungstate MWO4: M = bivalent (Ca, Ba, Mn, Co, Ni….etc.) have been sharply jumped in various application because of their extraordinary electronic, magnetic and catalytic properties. The MWO4 exist in tetragonal scheelite structure space group: I41/a, with Z = 4 and monoclinic wolframite structure space group: P2/c, with Z= 2, depends on M size. In the case of wolframite type structure, the divalent metal radii are smaller than 0.77 A (Mg2+, Fe2+, Co2+, Ni2+, and Zn2+). On the contrary, Scheelite type structure has greater ionic metals radius (0.77 A), the metal ion is as follow Ba2+, Ca2+, Sr2+, and Pb2+, where W atoms have six-fold coordination. Although there is an ample number of a report on wolframite structure metal tungstates such as MnWO4, FeWO4, CoWO4, NiWO4, and ZnWO4. Among them, a monoclinic wolframite structure nickel tungsten oxide (NiWO4) has remarkable attention owing to its widespread physicochemical properties. It has been reported in photocatalytic, supercapacitor, bio-sensor and in the electro-catalytic field. As stated in some previous report, the NiWO4 nano/microstructure including nano-bricks, fibers, nano-nests, and nanospheres was prepared through hydrothermal, solvothermal, electrospinning and solid-state reaction method for numerous application. For example, Shivakumar Mani et al successfully synthesized NiWO4 microcrystals completed octahedron structure by hydrothermal method, have stimulated excessive interest and significant bio-sensing activity towards glucose sensing. Shareen Fatima Anis et al reported electrospun NiWO4 composite fibers has good electrocatalyst hydrogen evolution performance compared with other metal oxide materials. Lengyuan Niu et al have studied asymmetric supercapacitor on NiWO4 nanostructure, have comparable electrochemical performance.


In this regards, surfactant-free self-growth assembly of the 3D NiWO4 may offer significant tool. This 3D flower-like structure of metal oxide have molded through continuous growth and Ostwald ripening, have shown significant electrochemical properties. Accordingly, we have reported an easy route chemical reduction method for the synthesis of 3D-NiWO4 micro flowers in an aqueous medium. Our optimization experimental condition have tuned the morphology evolution including nanoparticles, sheet, and micro flower. Therefore after analysis these critical issues, our aim to engineering controllable morphology of NiWO4 in 3D structure. We also elucidated growth mechanism and characterized as synthesized NiWO4 in detail by several techniques. In addition, the 3D NiWO4 microstructure has been applied to study electrochemical property for non-enzymatic glucose detection application.


2.         Experimental

2.1.      Preparation of 3D Nickel Tungsten Oxide flower

The 3D Nickel tungsten Oxide flowers were prepared surfactant free through chemical reduction method. Typically, 0.087 g Ni (NO3)2.6H2O (Sigma-Aldrich, 99.99 %) was dissolved in 30 ml deionized water and kept at 60 C temperature onto the hot plate. Followed by, 0.098 g Na2WO4.2H2O (Sigma-Aldrich 99.99%) was added after 10 minutes while being continuously stirring at 450 rpm. Wait till it became uniform solution and then added slowly 200 ml N2H4.H2O (Daejeon chemical 99.99 %) as a reducing agent, immediately solution became sky blue in color and left for 2 hours to complete reaction. The precipitates were collected by centrifugation and washed several times with water and ethanol before dried in Oven at 60 C. Finally, Nickel Tungsten Oxide was produced via calcination of the dried product at 800 C for 2 h in the box furnace. By varying the synthesis temperature, amount of reducing agent, molar ratio (Ni : W) and stirring rate, NiWO4 has different structures.

2.2.      Characterization

The surface morphologies and elemental composition of NiWO4 were characterized by field emission electron microscopy (FESEM, JEOL 7500F) along with an energy-dispersive X-ray spectroscope (EDS).  Powder X-ray diffraction was performed to check crystalline phase of the samples through X-ray diffraction (PDR-XRD, Bruker D8 FOCUS) using CuK? radiation at 5 s/step scan speed with 0.02° step size at 40 kV voltage and 40 mA current in the range between 10 to 70° with unlocked coupled scanning mode. The chemical bonding within the samples were recorded by Fourier transform infrared (FTIR) with (Bruker IFS-66/S) instrument at room temperature in the 550-4000 cm-1 region. The electrochemical experiment were recorded with CHI Models 660 C and 760D electrochemical workstations (USA) using three-electrode system contains modified of rotating disc glassy carbon electrode (GCE), saturated calomel electrode (SCE) and platinum wire serve as a working, reference and counter electrode respectively. The electrochemical properties of the sample modified GCE were measured through cyclic voltammetry (CV) technique.

2.3.      Preparation of 3D-NiWO4 flowers modified GCE

The modified 3D-NiWO4 GCE was prepared by making suspension of 5 mg as synthesized product into 5 ml ethanol with ultra-sonication assisted for 1 h. Before drop casting of 10 mL suspension of the prepared sample on to the GCE, the GCE has been polished out with alumina powder and washed with copious amount of water to remove impurities on the electrode surface. Then the modified electrode was dried at room temperature to use for electrochemical determination.


3.         Result and discussion


The NiWO4 micro flowers were synthesized surfactant free at low temperature through chemical reduction method by using N2H4 as a reducing agent, Na2WO4.2H2O for tungsten source and Ni(NO3)2.6H2O as the nickel source in the aqueous condition. The NiWO4 micro flowers were synthesized surfactant free at low temperature through chemical reduction method by using N2H4 as a reducing agent. Figure1. Shows the powder XRD patterns in the two theta scale 10-70° of the flower-like 3D-NiWO4 micro structured as synthesized material at 60° C with nickel/tungsten molar ratio 1:1 following 450 rpm kinetic range. However, flower-like 3D-NiWO4 microstructure exhibited several distinct diffraction peaks at 15.64°, 19.28°, 23.97°, 24.92°, 30.94°, 36.57°, 39.13°, 41.71°, 44.77°, 46.42°, 49.08°, 51.13°,  52.37°, 54.67°, 62.36°, 65.85°, and 69.01° which are allocated for (0 1 0), (1 0 0), (0 1 1), (1 1 0), (1 1 1), (0 2 1), (2 0 0), (1 0 2), (1 1 2), (2 1 1), (0 2 2), (2 2 0), (1 3 0), (2 2 1), (1 1 3), (0 2 3), and (0 4 1) planes respectively. The XRD patterns of wolframite monoclinic structure with unit cell of a = 4.60000, b = 5.66000, c = 4.91000 and space group of P2/c, which well match with standard data of NiWO4 (JCPDS no. 01-072-1189). In the XRD pattern, no other impurities peak implying the high crystallinity as well as pure phase formation of synthesized product. To be noted (111) plane has high intensity, reveled the crystal growth direction.



FTIR and Raman

The atomic vibration of flower-like 3D-NiWO4 microstructure was characterized using FTIR and Raman spectroscopy. The FTIR spectrum of the synthesized product at 400 – 4000 cm-1 as shown in figure 2A.

 These vibration modes are well matched with previously reported literature. The wolframite monoclinic NiWO4 crystal has18 vibration Raman active modes among 36 vibration modes, established by calculating group theory: I = 8Ag + 10Bg + 8Au + 10Bu. Figure 2B. shows the Raman bands at 889 cm-1, 769 cm-1, 693 cm-1, 542 cm-1, 509 cm-1 and 418 cm-1 respectively, specify the NiWO4 formation. Moreover, high intensity Raman band observed at 889 cm-1 illustrate W-O bond stretching vibration.



The morphology of as synthesized material were confirmed by SEM analysis. Figure 3A demonstrate the 3D microstructure of NiWO4 as well as uniform flower-like morphology under 3 micrometer size.  By monitoring easy route construction of morphology could not be ruined till the higher annealing temperature (800 C) indicating well defined microstructure. The high magnification SEM image in figure 3B revealed the flower-like morphology assembled through nanoparticles building block.

Effect of synthesis temperature

Temperature known as thermodynamics parameter play significant role to control the inorganic materials morphology. We have investigate the temperature effect during NiWO4 synthesis, while the stirring rate fixed at 450 rpm and 200 ml concentration of hydrazine. All XRD patterns of the different temperature (30, 60, 90 and 120 C) synthesis of NiWO4 monoclinic structure could achieved in Figure S1, specify the pure phase formation. Though, we found the random particles aggregation at 30 C and imperfect morphology at 120 C as well as formation of flower-like structure at 60 and 90 C. Compared to temperature synthesis higher peak intensity at 90 C temperature indicating the higher crystallinity along with uniform structure. Morphology of the sample tuned progressively as temperature increase from 30-120 C and crystal growth enlarged expressively. Nevertheless, the diffusion as well as migration rate of ion improved as the increase the synthesis temperature and growing micro flower rapidly along <111> directions. All the same, rising temperature will outcome the more segmentation of flower surface and go on the damaged flower morphology with intense adhesion.   

Effect of reducing agent

In the previous report, Zhang and many other researcher have described hydrothermal method and assistance of surfactant to control the microstructure, which is reasonably tough procedure to find out selective surfactant and high temperature synthesis. In this context, flower-like NiWO4 microstructures could become at optimize temperature with the help of hydrazine as a strong reducing agent. The 3D flower-like NiWO4 microstructure obtained upon assisting hydrazine. On the contrary, imperfect agglomerated nanoparticles obtained without hydrazine. Thus, we have notified the roll of hydrazine in the synthesis route provide the essential impact on NiWO4 morphology and introducing concentration of this agent upon the NiWO4 synthesis have been considered sensibly. The NiWO4 synthesis thermodynamically fixed at 60 C along with kinetic rate at 450 rpm, while concentration of hydrazine is varied from 50-200 ml.













Electrochemical behavior

Electrochemical behavior was performed by Cyclic Voltammetry three electrode system involving GCE working electrode, a Pt wire counter electrode and a Hg/HgO reference electrode. The making solution of flower-like 3D NiWO4 was drop cast onto the GCE, which was predicted for cyclic performance. The CV test of flower-like 3D-NiWO4 modified GCE illustrated with 10 to 100 mVs-1 scan rate over potential window range 0 to 0.6 V in the 0.1 M NaOH electrolyte, have seen in figure 8A. We found the regular increment of cathodic and anodic current along with their peak-to-peak deference with rising scan rate. Figure 8B. shows the cathodic peak linearity with increasing scan rate revealed the existence of a surface-controlled electrochemical behavior. Of particular note, the contribution of an oxyanion WO42- in NiWO4 composition assistance to encourage greater electrical conductivity. On the contrary, NiO play a significant role while redox reaction, which mechanism explained by wang et al. It was observed the flower-like 3D-NiWO4 has good electrochemical behavior and may utilize for the electrochemical-based application.












4.         Conclusion

In this paper, we synthesized a facile surfactant free controllable 3D flower-like morphology of hierarchical NiWO4 through chemical reduction method. It could be seen that the uniformed NiWO4 microstructures were accumulated by very pure monoclinic phase of NiWO4 nanoparticles. It is confirmed that change the reaction condition, such as stirring rate, temperature and the addition of reducing agent, all have the individual impotency to control the morphology of NiWO4 material.






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