Microwave and hydrothermal syntheses of WSe₂ micro/nanorods and their application in supercapacitors

Abstract

WSe₂ micro/nanorods were synthesized using one step microwave and hydrothermal methods. The as synthesized micro/nanorod samples of WSe₂ were characterized using various characterization techniques such as SEM, TEM, Raman spectroscopy, X-ray diffraction, UV-visible and PL spectroscopy. The as synthesized samples were also tested for their applicability to use as cathode materials for supercapacitor applications. The WSe₂ samples prepared by the microwave and hydrothermal methods (with use of tungstic acid as precursor) show noteworthy performance towards supercapacitor application. Our work opens a new avenue to use these simple methods to prepare various morphologies of inorganic nanomaterials and utilize them for various energy and nanoelectronics applications.

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Introduction

Supercapacitors, which are also known as electrochemical capacitors, are considered as essential power sources due to promising energy-storage device applications. The important parameters of these devices are high power densities, rate of charge–discharge, life cycle stability, durability and low cost. Owing to these remarkable properties, researchers have been attracted to the area of supercapacitors, to fabricate electrodes using various nanomaterials for advanced energy storage devices. Over the past few years, there has been remarkable growth in various perspectives of engineering applications of one-dimensional (1D) and two-dimensional (2D) inorganic oxide layered materials especially transition metal dichalcogenides^1–7 such as MoS₂,^8–24 WS₂,^25–28 MoSe₂,^29–32 WSe₂,^33–39 GaS,^15,40 GaSe^15,40 and other 2D insulating^41–44 and oxide materials^45,46 which have attracted much attention from the scientific community due to their extraordinary electrical, optical and magnetic properties. Among all, the WSe₂ layered materials in 1D nanometric form have not yet been widely investigated for energy storage and other applications due to difficulties in the synthesis and other related issues. The 2D form of WSe₂ has been recently reported as field effect transistors,^8–13 photodetectors,¹⁶ for photocatalytic hydrogen production⁴⁷ in solar cells^48–50 and heterostructures.¹¹ The bulk WSe₂ is a layered semiconductor with indirect bandgaps ∼1.21 eV,⁵¹ in monolayer form it exhibits direct bandgaps of 1.67 eV.⁵² The WSe₂ belongs to the transition metal dichalcogenides (TMDCs) crystal structure made up of hexagonal layers of metal atoms (M = Mo, W, V, Ga, Ta, Sn) sandwiched in between two layers of chalcogen atoms (X = S, Se, Te).^8–24 The WSe₂ is a naturally occurring metal selenide with intriguing electronic, electrochemical and electrocatalytic properties, formed by 2D covalently bonded Se–W–Se layers separated by a van der Waals gap. The WSe₂ possesses a hexagonal crystal structure with space group P63/mmc and each WSe₂ monolayer contains an individual layer of W atoms with 6-fold coordination symmetry, which is hexagonally packed between two trigonal atomic layers of Se atoms (Fig. 1).

Fig. 1 Schematic of WSe₂ structure: (a) side view and (b) top view.

The atomically thin layered WSe₂ has attracted much attention for diverse applications in nanoelectronic devices because of its suitable wide and direct band gap.⁵³ It is possible that the layered WSe₂ can offer easy electron transport through MX₂ nanostructures, which gives high specific capacitance values by faster ion diffusion between the layers as reported for other TMDCs.^54,55 Graphene and its composites with other materials have been utilized for its suitability for supercapacitor applications.^54,55 Moreover it is important that for high performance supercapacitors, materials should have properties such as high power density, fast power delivery or uptake, and excellent cycle-to-cycle stability etc. These properties are essential for material to be used in high performance energy storage devices.^56–61 The TMDCs materials like MoS₂ and WS₂ have been recently reported for their supercapacitor behaviour,^54,55 but WSe₂, which also belongs to the same layered chalcogenide family, has not yet been explored till date for its application as a supercapacitor.

In the present investigations, we report the synthesis of WSe₂ nanorods and nanoparticles using a simple microwave assisted route and a hydrothermal method and their supercapacitor behaviours. The WSe₂ exhibited enhanced and stable supercapacitive behaviour in three electrode capacitance measurement geometry.

Results and discussion

The WSe₂ samples were prepared using three different routes. Here after the sample prepared by the microwave-assisted method is named as WSe₂-A, next the sample prepared using the hydrothermal method using tungstic acid and elemental selenium salts is called as WSe₂-B and finally the sample prepared using the hydrothermal method with ammonium tungstate and elemental selenium salts is called as WSe₂-C. Here after samples are described by above nomenclature. Fig. 2(a–d) shows the typical SEM images of the WSe₂-A sample prepared using the microwave method depicting the rod-like morphology with typical dimensions 10–80 μm in length and 1–2 μm in diameter. Fig. 3(a–d) shows the typical TEM images of the WSe₂-A sample, depicting the rod-like morphology. Fig. 3(e and f) shows HR-TEM images of the WSe₂-A sample showing the good quality of the samples prepared using the microwave route. The inset of Fig. 3(f) shows the typical diffraction pattern of the WSe₂-A sample, showing hexagonal structure and highly oriented along the <001> axis. Fig. 4(a) shows the typical XRD pattern of the WSe₂-A sample, which matches well with the JCPDS data card no 06-0080. The sample also shows some other phases in addition to WSe₂ which indicates that our sample is slightly impure. This needs additional detailed X-ray photoelectron spectroscopy and other investigations. The WSe₂-A sample prepared using the microwave method has been further characterized using Raman spectroscopy as shown in Fig. 4(b). The typical Raman spectrum of the WSe₂-A sample excited using a 514.5 nm laser source shows Raman bands at A_1g-LA (142 cm⁻¹), E_2g¹ (240 cm⁻¹). It has been reported that the A_1g and E_2g¹ bands of single-layer WSe₂ are separated by ∼11 cm⁻¹.⁵⁴ The light absorption properties of the as-synthesized WSe₂-A sample were characterized by the UV-Vis absorption spectrum. Fig. 4(c) shows the UV-Vis absorption spectrum of WSe₂-A. The absorption of WSe₂-A was extended to the UV region. From the optical absorption spectrum, it is evident that the WSe₂-A showed an absorption onset of 220 nm. The room temperature PL spectrum of the WSe₂-A sample is shown in Fig. 4(d). It exhibits a strong PL peak at ∼728 nm.

Fig. 2 (a–d) SEM images of WSe₂-A synthesized by the microwave method using ammonium molybdate salt.

Fig. 3 (a–d) TEM images of WSe₂ synthesized by the microwave method. (e–f) HRTEM images and inset of (f) shows the typical SAED pattern showing the high crystalline quality of the as synthesized sample.

Fig. 4 WSe₂ microrod sample synthesized by the microwave method. (a) XRD, (b) Raman spectrum, (c) UV-visible spectrum and (d) PL spectrum.

The supercapacitive performances of the WSe₂-A samples were measured in a three-electrode system in 0.5 M KOH aqueous electrolyte. The specific capacitances of the WSe₂ electrode have been calculated from the following equation:

where m is the mass of single electrode material, v is the scan rate (mV s⁻¹), ν1 and ν2 are the integration limits (potential window), and I(ν) denotes the response current (A). The capacitive performance of WSe₂-A was measured in a three-electrode system in 0.5 M KOH aqueous electrolyte. Fig. 5(a) shows the comparative cyclic voltammograms (CVs) of WSe₂-A at different scan rates. The CV curve consists of an area under the curve which shows the charge storage capacity of the electrode therefore the value of specific capacitance calculated at a scan rate 100 mV s⁻¹ is found to be 1.939 F g⁻¹. Fig. 5(b) shows the typical comparative charge–discharge profiles of WSe₂-A at current densities of 1 and 5 μA. Fig. 5(c) depicts the single charging–discharging cycle for the 400 cycles. The electron/ion transport at the electrode/electrolyte interface and charge transfer resistance (R_ct) is further investigated by electrochemical impedance spectroscopy (EIS) techniques; the Warburg impedance can be determined using the straight line inclined to the real axis.⁶² The slope of the Warburg impedance depicts the diffusion of electrolyte into the electrodes. The R_ct values of WSe₂-A samples were measured in 0.5 KOH at 10.0 Ω which is shown in Fig. 5(d).

Fig. 5 Supercapacitor based on WSe₂ microrods synthesized by the microwave method. (a) Cyclic voltammograms at different scan rates, (b) charging–discharging profiles at different current densities, (c) charging–discharging profile for 400 cycles and (d) impedance spectroscopy.

Fig. 6(a)–(e) shows the typical SEM images of the WSe₂-B sample depicting microrod-like morphology which is similar to WSe₂-A, with dimensions 10–90 μm in length and 1–2 μm diameters. Fig. 7(a)–(c) shows typical TEM images of the WSe₂-B sample and Fig. 7(d) shows the typical SAED pattern of the sample representing the single crystalline nature of the WSe₂-B sample. Fig. 8(a) shows the typical XRD pattern of WSe₂-B which matches with the JCPDS data card no. 06-0080. Fig. 8(b) depicts the typical Raman spectrum of the WSe₂-B sample which matches with reported spectra in the literature.⁵⁶ The light absorption study was carried out using the UV-Vis absorption spectrum and is shown in Fig. 8(c). The absorption of WSe₂-B was observed in the UV region and the absorption onset of 220 nm was noted. The PL spectrum of the WSe₂-B sample was taken at room temperature as shown in Fig. 8(d). It exhibits a strong PL peak centred at 728 nm. The capacitive performance of WSe₂-B was also investigated in a three-electrode system in 0.5 M KOH aqueous electrolyte. Fig. 9(a) shows the comparative cyclic voltammograms (CVs) of the WSe₂-B sample at different scan rates. The area under the CV curve represents the charge storage capacity of the electrode therefore the value of specific capacitance calculated at a scan rate of 100 mV s⁻¹ is found to be 2.44 F g⁻¹. The comparative charge–discharge profiles of WSe₂-B at different current densities are showed in Fig. 9(b). Fig. 9(c) depicts the single charging–discharging cycle from the 400 cycles which is shown in Fig. 9(d). The R_ct values of WSe₂-B nanorods were measured in 0.5 KOH at 11.0 Ω.

Fig. 6 (a–f) SEM images of WSe₂ microrods synthesized by the hydrothermal method using tungstic acid as source material.

Fig. 7 (a–c) TEM images of a WSe₂ microrod synthesized by the hydrothermal method and (d) typical SAED pattern of the WSe₂ micro/nanorod sample.

Fig. 8 WSe₂ micro/nanorods synthesized by the hydrothermal method using tungstic acid as the source. (a) XRD pattern, (b) Raman spectrum, (c) UV-visible spectrum and (d) PL spectrum.

Fig. 9 Supercapacitor based on WSe₂ micro/nanorod sample synthesized by the hydrothermal method using tungstic acid as source: (a) cyclic voltammograms at different scan rates, (b) charging–discharging profiles at different current densities, (c) charging–discharging profile for 400 cycles and (d) impedance spectroscopy.

Fig. 10(a)–(f) shows the typical SEM images of the WSe₂-C sample, showing micron size particle-like morphology with dimensions 2–5 μm which were further observed to be decorated with small nanoparticles on the micron size particle. Fig. 11(a) and (b) depicts typical TEM images of the WSe₂-C sample, which shows good uniform growth of particle-like morphology. Fig. 11(c) and (d) shows HR-TEM images of the WSe₂-C sample in high resolution. The crystal qualities of the samples were analysed using the diffraction pattern which is shown in the inset of Fig. 11(d). Fig. 12(a) shows the typical XRD pattern of the WSe₂-C sample confirming the high quality of the as synthesized sample. The WSe₂-C sample was further examined by Raman spectroscopy. Fig. 12(b) shows the Raman spectrum of WSe₂-C matches very well with the earlier results in this manuscript. The light absorption characteristics were examined using the UV-Vis absorption spectrum which is shown in Fig. 12(c). The absorption of the WSe₂-C sample was characterized using UV absorption with an observed onset of 220 nm. The WSe₂-C sample was further characterized by the PL spectrum at room temperature, it exhibits a strong PL peak centred at 727.9 nm, depicted in Fig. 12(d). The capacitive performance of WSe₂-C was also measured in a three-electrode system in 0.5 M KOH aqueous electrolyte. Fig. 13(a) shows the comparative cyclic voltammograms (CVs) of WSe₂-C at different scan rates. The area under the CV curve represents the charge storage capacity of the electrode therefore the value of specific capacitance calculated at a scan rate of 100 mV s⁻¹ is found to be 2.59 F g⁻¹. Fig. 13(b) shows the typical comparative charge–discharge profiles of the sample at different current densities. Fig. 13(c) depicts the single charging–discharging cycle from the 400 cycles. Fig. 13(d) shows the impedance spectrum of WSe₂-C. The results typically consist of a curve at high frequencies and a straight line at lower frequencies. The R_ct values of WSe₂-C were measured in 0.5 KOH at 37.0 Ω. Fig. 13(e) shows the comparative CVs of all three sample WSe₂-A, WSe₂-B and WSe₂-C at 100 mV s⁻¹ and Fig. 13(f) depicts the comparative charge–discharge profiles at the same current density of 5 μA. Table 1 summarizes the results of supercapacitor performances at different voltage sweep rates (50, 100, 200, 500, and 1000 mV s⁻¹) showing the specific capacitances which were examined for all three samples of WSe₂.

Fig. 10 (a–f) SEM images of WSe₂ micro/nanoparticles synthesized by the hydrothermal method using ammonium molybdate as source material.

Fig. 11 WSe₂ micro/nanoparticles synthesized by the hydrothermal method using ammonium molybdate as source: (a–b) TEM images, (c–d) HR-TEM images. Inset of (d) shows the typical SAED pattern of WSe₂ micro/nanoparticles.

Fig. 12 WSe₂ micro/nanoparticles synthesized by the hydrothermal method using ammonium molybdate as source material: (a) XRD pattern, (b) Raman spectrum, (c) UV-visible spectrum and (d) PL spectrum.

Fig. 13 Supercapacitor based on WSe₂ micro/nanoparticles synthesized by the hydrothermal method using ammonium molybdate. (a) Cyclic voltammograms at different scan rates, (b) charging–discharging profiles at different current densities, (c) charging–discharging profile for 400 cycles and (d) impedance spectroscopy; (e and f) show the comparative study of WSe₂-A, WSe₂-B and WSe₂-C samples for cyclic voltammograms at 100 mV s⁻¹ and charge–discharge profiles at a current density of 5 μA, respectively.

Table 1 Specific capacitance for all samples of WSe₂ at different scan rates

Sample	Voltage scan rate 10 mV s^−1	Voltage scan rate 50 mV s^−1	Voltage scan rate 100 mV s^−1	Voltage scan rate 200 mV s^−1	Voltage scan rate 500 mV s^−1	Voltage scan rate 1000 mV s^−1
WSe2-A	10.41	2.48	1.939	1.58	1.26	1.06
WSe2-B	10.41	3.76	2.44	1.50	0.88	0.64
WSe2-C	6.125	3.19	2.59	2.12	1.61	1.28

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Conclusions

In conclusion, we have synthesized WSe₂ micro/nanorods using two different methods namely one step microwave and hydrothermal methods. We have also successfully synthesized WSe₂ micro/nanoparticles covered with small WSe₂ nanoparticles (size ∼ 100 nm) using the hydrothermal method. In addition to pure phase some impurities in the sample were also noted, which need further detailed investigations. The samples were tested for their suitability to use in supercapacitor applications. The WSe₂ microrods prepared using the microwave and hydrothermal methods show better performance towards cyclic stability and good performance towards charging and discharging cycles. Impedance spectroscopy results of all samples reveal that a lower R_ct value and thicker electrical double layer formations are responsible for its superior capacitive behaviour. Our results open a new avenue to prepare WSe₂ and various other inorganic layered materials using simple hydrothermal and microwave methods and the use of novel morphologies of these materials for energy applications including supercapacitors to replace conventional batteries.

Experimental section

(a) Microwave synthesis of WSe₂ nanorods

In the typical microwave reaction the stoichiometry quantities of the precursor powders elemental selenium (2 mM) and ammonium tungstate (1 mM) and sodium borohydride (3 mM) were dissolved in 10 mL ethylene glycol followed by purging inert N₂ gas for a few minutes prior to switching on the microwave. The microwave-assisted reaction was carried out in a Spectra 750 W microwave oven, with a 2.45 GHz working frequency. The oven was modified to include a refluxing system. During the experiments, the microwave oven was operated for switching on for 30 s and off for the next 15 s, with use of total power ∼750 W up to 3 minutes. For the post reaction, the product was centrifuge using ethanol solvent for several times at 4500 rpm followed by washing using DI water and drying in vacuum for 3 hours.

(b) Hydrothermal synthesis of WSe₂ microrods

The second method for WSe₂ microrod synthesis was the hydrothermal reaction. In a typical reaction, we used 1 g of ammonium tungstate [(NH₄)₁₀H₂(W₂O₇)₆], 0.89 g of elemental selenium and 8 mL of hydrazine monohydrate which were added in a 25 mL capacity Teflon-lined stainless steel autoclave and distilled water was added to fill 80% of the total volume of the autoclave. In the hydrothermal reaction, typical temperature was maintained at 150–170 °C for 48 h.

(c) Hydrothermal synthesis of WSe₂ nanoparticles

The third method to synthesize WSe₂ nanostructures was the same as the hydrothermal method with a different source of tungsten as tungstic acid was used. In the hydrothermal method, the typical temperature of Teflon-lined steel autoclave was maintained at 150–170 °C for 48 h. For the post reaction the autoclaves were cooled naturally followed by washing with distilled water and ethanol successively and then the final product was dried under vacuum at 50 °C for 2 h.

(d) Characterization

All WSe₂ samples were further characterized using several analytical tools such as XRD, Raman, SEM, TEM, UV-Vis and PL etc. The crystallographic structures and phase confirmation of WSe₂ samples were analysed by X-ray diffraction (XRD, Philips powder diffractometer PW3040/60 with Cu Kα radiation). Scanning electron microscopy (SEM) images were acquired using an FEI ESEM QUANTA 200 3D instrument. High resolution-transmission electron microscopy (HR-TEM) images were obtained using an FEI TECNAI TF-30 (FEG) instrument. The Raman spectra were recorded at room temperature, with a (LabRAM HR) using an Ar laser (514.5 nm) in the back scattering geometry. The adsorption capacities of the samples were analysed using a HORIBA Fluorolog 3 spectrofluorometer instrument. The photoluminescence was measured by exciting the samples with a 400 nm solid-state laser of few milliwatts power.

Acknowledgements

Dr D. J. Late would like to thank Department of Science and Technology (Government of India) for Ramanujan Fellowship (Grant no. SR/S2/RJN-130/2012). The research work was primarily supported by NCL-MLP project grant 028626, DST-SERB Fast-track Young scientist project Grant no. SB/FT/CS-116/2013 and the partial support by INUP IITB project sponsored by DeitY, MCIT.

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