**3.2 Urchin-like ZnS/Ni3S2@Ni composites as microwave absorbers**

The EM absorption properties of nanomaterial are associated with the size, shape, and dimensionality. Metal sulfide nanomaterials have captured more and more attention due to their excellent properties and wide applications in electronic and optoelectronic devices. Nickel subsulfide and ZnS are the important categories in the metal sulfide family [52, 53] thanks to their various applications such as in lithium ion batteries, supercapacitors, dye-sensitized [54, 55] and charge transfer, anion exchange, electricity generation [56, 57], respectively. Due to the semiconductor properties of nickel subsulfide and ZnS, it can induce dipole and space charge polarizations when placed in the alternated electromagnetic field [16, 58]. Moreover, nickel subsulfide (Ni3S2) are highly metallic compared to insulating oxide compounds, which can cause conductive loss [59]. The core-shell Ni@ZnS composites with the improvement of electromagnetic properties were reported by our earlier literatures [45, 60]. To the best of our knowledge, the EM wave absorption of core-shell structured ternary ZnS/Ni3S2@Ni composite is hardly found in the published papers. Herein, a novel ZnS/Ni3S2@Ni composite with urchin-like core-shell structure is successfully synthesized, and it exhibits the excellent EM absorption and the absorption mechanism of such unique hierarchical microstructure is also discussed in detail.

## *3.2.1 Preparation of urchin-like ZnS/Ni3S2@Ni composites*

The monodispersed Ni microspheres were prepared according to our previous literatures [37, 45, 62]. Synthesis of urchin-like core-shell structured ZnS/Ni3S2@Ni composite [61]: in brief, Ni (0.05 g) was dispersed in a mixture of aqueous solution of distilled water (30 mL) and ethanol (30 mL) containing 1.0 M NaOH. Then, 1 mmol ZnCl2 and 2 mmol Na2S·9H2O were introduced into above mixture, respectively. The mixture was transferred to a Teflon-lined autoclave. The sealed autoclave was heated to 120°C for 15 h. To explore the possible generation mechanism of coreshell microstructure urchins and effects of temperatures on the shapes of target products, the temperature-control experiments (60°C, 80°C, 100°C and 120°C) were also conducted.

The phase constituent and structure of the as-prepared Ni microspheres and urchin-like ZnS/Ni3S2@Ni products are characterized by XRD. **Figure 11a** displays the XRD curve of uncoated Ni microspheres, which could be well assigned to facecentered cubic structure of Ni (JCPDS No. 040850). As presented in **Figure 11b**, except for the diffraction peaks of Ni, the other diffraction peaks could be attributed to the zinc-blende ZnS (JCPDS Card No. 05-0566) and Ni3S2 (JCPDS Card No. 76-1870). From these XRD patterns, it can be confirmed that the core-shell ZnS/Ni3S2@Ni composite is composed of nickel, nickel sulfide and zinc sulfide. It can be inferred that nickel has functioned as the template for in-situ generation of Ni3S2 and deposition of ZnS.

To investigate the morphology of the products, FESEM images are taken for Ni microspheres and ZnS/Ni3S2@Ni composite and the corresponding results were shown in **Figure 12**. From the **Figure 12a**, it can be seen that Ni products were composed of uniform distribution and smooth surface of microspheres. **Figure 12b, c** presents the different magnification FESEM images of core-shell ZnS/Ni3S2@Ni composite. Interestingly, from panoramic observation (**Figure 12b**), urchin-like products are optionally grown on the surfaces of Ni microspheres and the outline of Ni spheres cannot be clearly observed due to the formation of ZnS/Ni3S2. Noticeably, the decease size of Ni particles could be observed, which suggests the depletion of Ni products. The thorns possess about 1 μm and 50 nm in length and

**165**

**Figure 12.**

**Figure 11.**

*Electromagnetic Wave Absorption Properties of Core-Shell Ni-Based Composites*

diameter, respectively. Further observation from the high magnification FESEM image (**Figure 12c**), the existence of crumple products encircle Ni particles and the crinkled products are expected to be linked between Ni particles and thorns. **Figure 12d** presents the EDS of the ZnS/Ni3S2@Ni composite. The EDS reveals the presence of elements of S, Zn and Ni. Pt peaks are also seen in the EDS curve because the SEM

*FESEM images of (a) Ni microspheres, (b, c) urchin-like core-shell structured ZnS/Ni3S2@Ni composite, and* 

*XRD patterns of (a) pristine Ni microspheres and (b) as-prepared ZnS/Ni3S2@Ni [61] (permission from RSC).*

It is assumed that the reaction temperature has an effect on the morphology of core-shell heterostructure. At low temperature (60°C), interestingly, there are plentiful waxberry-like products existed in **Figure 13a**. It is due to the fact that Ni

sample is prepared by sputtering of platinum onto the sample.

*(d) EDS profile of ZnS/Ni3S2@Ni composite [61] (permission from RSC).*

*DOI: http://dx.doi.org/10.5772/intechopen.82301*

*Electromagnetic Wave Absorption Properties of Core-Shell Ni-Based Composites DOI: http://dx.doi.org/10.5772/intechopen.82301*

**Figure 11.** *XRD patterns of (a) pristine Ni microspheres and (b) as-prepared ZnS/Ni3S2@Ni [61] (permission from RSC).*

#### **Figure 12.**

*Electromagnetic Materials and Devices*

discussed in detail.

were also conducted.

Ni3S2 and deposition of ZnS.

**3.2 Urchin-like ZnS/Ni3S2@Ni composites as microwave absorbers**

*3.2.1 Preparation of urchin-like ZnS/Ni3S2@Ni composites*

The EM absorption properties of nanomaterial are associated with the size, shape, and dimensionality. Metal sulfide nanomaterials have captured more and more attention due to their excellent properties and wide applications in electronic and optoelectronic devices. Nickel subsulfide and ZnS are the important categories in the metal sulfide family [52, 53] thanks to their various applications such as in lithium ion batteries, supercapacitors, dye-sensitized [54, 55] and charge transfer, anion exchange, electricity generation [56, 57], respectively. Due to the semiconductor properties of nickel subsulfide and ZnS, it can induce dipole and space charge polarizations when placed in the alternated electromagnetic field [16, 58]. Moreover, nickel subsulfide (Ni3S2) are highly metallic compared to insulating oxide compounds, which can cause conductive loss [59]. The core-shell Ni@ZnS composites with the improvement of electromagnetic properties were reported by our earlier literatures [45, 60]. To the best of our knowledge, the EM wave absorption of core-shell structured ternary ZnS/Ni3S2@Ni composite is hardly found in the published papers. Herein, a novel ZnS/Ni3S2@Ni composite with urchin-like core-shell structure is successfully synthesized, and it exhibits the excellent EM absorption and the absorption mechanism of such unique hierarchical microstructure is also

The monodispersed Ni microspheres were prepared according to our previous literatures [37, 45, 62]. Synthesis of urchin-like core-shell structured ZnS/Ni3S2@Ni composite [61]: in brief, Ni (0.05 g) was dispersed in a mixture of aqueous solution of distilled water (30 mL) and ethanol (30 mL) containing 1.0 M NaOH. Then, 1 mmol ZnCl2 and 2 mmol Na2S·9H2O were introduced into above mixture, respectively. The mixture was transferred to a Teflon-lined autoclave. The sealed autoclave was heated to 120°C for 15 h. To explore the possible generation mechanism of coreshell microstructure urchins and effects of temperatures on the shapes of target products, the temperature-control experiments (60°C, 80°C, 100°C and 120°C)

The phase constituent and structure of the as-prepared Ni microspheres and urchin-like ZnS/Ni3S2@Ni products are characterized by XRD. **Figure 11a** displays the XRD curve of uncoated Ni microspheres, which could be well assigned to facecentered cubic structure of Ni (JCPDS No. 040850). As presented in **Figure 11b**, except for the diffraction peaks of Ni, the other diffraction peaks could be attributed to the zinc-blende ZnS (JCPDS Card No. 05-0566) and Ni3S2 (JCPDS Card No. 76-1870). From these XRD patterns, it can be confirmed that the core-shell ZnS/Ni3S2@Ni composite is composed of nickel, nickel sulfide and zinc sulfide. It can be inferred that nickel has functioned as the template for in-situ generation of

To investigate the morphology of the products, FESEM images are taken for Ni microspheres and ZnS/Ni3S2@Ni composite and the corresponding results were shown in **Figure 12**. From the **Figure 12a**, it can be seen that Ni products were composed of uniform distribution and smooth surface of microspheres. **Figure 12b, c** presents the different magnification FESEM images of core-shell ZnS/Ni3S2@Ni composite. Interestingly, from panoramic observation (**Figure 12b**), urchin-like products are optionally grown on the surfaces of Ni microspheres and the outline of Ni spheres cannot be clearly observed due to the formation of ZnS/Ni3S2. Noticeably, the decease size of Ni particles could be observed, which suggests the depletion of Ni products. The thorns possess about 1 μm and 50 nm in length and

**164**

*FESEM images of (a) Ni microspheres, (b, c) urchin-like core-shell structured ZnS/Ni3S2@Ni composite, and (d) EDS profile of ZnS/Ni3S2@Ni composite [61] (permission from RSC).*

diameter, respectively. Further observation from the high magnification FESEM image (**Figure 12c**), the existence of crumple products encircle Ni particles and the crinkled products are expected to be linked between Ni particles and thorns. **Figure 12d** presents the EDS of the ZnS/Ni3S2@Ni composite. The EDS reveals the presence of elements of S, Zn and Ni. Pt peaks are also seen in the EDS curve because the SEM sample is prepared by sputtering of platinum onto the sample.

It is assumed that the reaction temperature has an effect on the morphology of core-shell heterostructure. At low temperature (60°C), interestingly, there are plentiful waxberry-like products existed in **Figure 13a**. It is due to the fact that Ni

#### **Figure 13.**

*FESEM images of core-shell ZnS/Ni3S2@Ni composites prepared at various reaction temperatures: (a) 60°C, (b) 80°C, (c) 100°C, and (d) 120°C [61] (permission from RSC).*

microspheres were coated by wrinkle ZnS products. Ni3S2 nanoparticles are insitu formed by depletion of Ni particles and then covered on the surface of left Ni products. With elevating the reaction temperatures to high temperatures (80°C), one can notice that some protuberant prickles were grown on the surfaces of core-shell composites (**Figure 13b**). When the reaction temperature is increased to 100°C, the presence of more and strong protuberant stabs on the crumple surfaces of composite can be obviously observed (**Figure 13c**). With further enhancing the temperature to 120°C, the target urchin-like core-shell structural ZnS/Ni3S2@Ni composites are formed with numerous of thorns or rods grown on the rugged surfaces (**Figure 13d**).
