Materials Characterization 171 (2021) 110741

Available online 4 November 20201044-5803/© 2020 Elsevier Inc. All rights reserved.

Significant recycled efficiency of multifunctional nickel molybdenum oxide nanorods in photo-catalysis, electrochemical glucose sensing and asymmetric supercapacitors

Jahangeer Ahmed a,*, Mohd Ubiadullah a, M.A. Majeed Khan b, Norah Alhokbany a, Saad M. Alshehri a

a Department of Chemistry, College of Science, King Saud University, Riyadh, Saudi Arabia b King Abdullah Institute for Nanotechnology, King Saud University, Riyadh 11451, Saudi Arabia

A R T I C L E I N F O

Keywords: Nanorods Recycled Photocatalysis Degradation Sensing Supercapacitor

A B S T R A C T

In this paper, we report significant recycled efficiency of nickel molybdenum oxide (NMO) nanorods for multi- functional catalytic activities in photo-catalysis, electrochemical glucose sensing and asymmetric super-capacitors. Initially, we have used NMO nanorods as the photo-catalysts for organic dye degradation reactions using visible light source. The NMO nanorods show momentous photocatalytic activity in degradation of organic dye (~90% in 160 min) and also exhibit excellent regeneration efficiency. Consequently, the photo-catalysts were recovered via batch experiments for further use in other applications including electrochemical glucose sensing and asymmetric supercapacitors. Powder X-ray diffraction (XRD) and electron microscopic experiments were conducted before and after photo-catalysis for the materials stability. The recovered NMO nanorods show significant electrochemical detection of glucose in 0.1 M KOH. Moreover, the recovered NMO nanorods show promising nature for supercapacitors. The specific capacitance of recovered NMO nanorods was found to be ~660 F/g at 2 mV/s in 5 M KOH. Charge-discharge and stability of the electrodes were also studied for the commercial purpose.

1. Introduction

The recycling process of the catalysts is the current interest of the researchers in commercial feasibility to the multifunctional catalytic activities. Currently, inorganic solid waste materials are demolished around the world with very high efficiency without knowing their re- cyclic efficiencies in diverse technologies. Herein, we are focused on the reusable efficiency of inorganic nanostructured materials for multipurpose applications. The recovery of catalysts for further use in various technologies can be considered as one of the cost effective ap-proaches. The compounds of group VI-B family (e.g. molybdenum and tungsten) are reported as efficient catalysts for attractive photochemical [1–6] and electrochemical applications [7–10]. Recently, nanocrystal-line NiMoO4 particles have shown promising interest in electrolysis of water for oxygen and hydrogen evolution reactions [11–17]. Jin-Sung Park et al. have reported NiMoO4 microspheres as anode materials in Li-ion batteries [18]. NiMoO4 nanorods have shown potential in energy- sustainable development via urea oxidation reactions [19–21].

Nanocrystalline NiMoO4 particles have shown potential in photo- catalysis with band gap energy of ~2.23 eV and can be used effec-tively as visible light photo-catalysts in the degradation of methylene blue dye [22,23], malachite green [24] and 4-chlorophenol [25]. NiMoO4 nanoparticles have also shown the catalytic activity in the reduction of nitro-phenol to the amino groups under UV–visible light [26]. Moreover, NiMoO4 nanostructured materials were used as sensors in H2O2 [27] and glucose detection [28,29].

Nanocrystalline binary transition metal oxides exhibit good capaci-tive behavior compared to other oxides due to variable oxidation states and good electrical conductivity. Among the binary oxides, NiMoO4 nanostructured materials are extensively recognized as low cost elec-trode materials with high electrical conductivity and excellent capaci-tive performance for energy storage devices [30,31]. Previously, variety of significant nanostructured materials has also been reported for energy storage applications [32–35]. High electrical conductivity of nickel molybdate arises from molybdenum and coupled with nickel to perform significant electrochemical behavior for admirable asymmetric

* Corresponding author. E-mail address: jahmed@ksu.edu.sa (J. Ahmed).

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Materials Characterization

journal homepage: www.elsevier.com/locate/matchar

https://doi.org/10.1016/j.matchar.2020.110741 Received 27 January 2020; Received in revised form 30 September 2020; Accepted 1 November 2020

Materials Characterization 171 (2021) 110741

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supercapacitors [31,36,37]. Solvothermally synthesized NiMoO4 nano-rods (diameter of ~0.1–0.3 μm) delivered significant specific capaci-tance of ~815 F/g at 1 A/g with the retention capacity of ~70% for 1000 cycles [38]. Ni-Co-S nanosheet supported NiMoO4/CoMoO4 nanorod showed specific capacitance of ~778 F/g at 0.5 A/g in 1 M KOH [39]. Hydrothermally synthesized NiMoO4 nanorods (diameter of ~30–130 nm) performed well with specific capacitance of ~590 F/g at 1 A/g in 6 M KOH [40]. Herein, we report the visible light photocatalytic activities of NiMoO4 nanorods in the degradation of organic dyes. Subsequently, the NiMoO4 photo-catalysts were recovered through batch process and further used in electrochemical glucose sensing and asymmetric supercapacitors in alkaline medium. The stability of cata-lysts has also been studied during photocatalytic and electrochemical measurements.

2. Experimental

NMO nanorods were synthesized from the molten salts as reported previously [15]. The prepared NMO nanorods were also characterized properly using the powder x-ray diffraction (BrukerD-8 Advanced diffractometer), Raman (Bruker Sentera Raman microscope), FTIR (Bruker TENSOR-27 spectrometer), scanning electron microscopy (JEOL-JSM-7600F) and transmission electron microscopy (JEOL-JSM- 2100F). After successful characterization of the NMO nanorods, we have carried out the photo-catalytic degradation reactions of methylene blue (MB) dye using visible light source. 50 mg NMO photo-catalyst was mixed with 100 mL of 10 mg/L methylene blue solution under constant stirring to make a proper suspension for adsorption/desorption equi-librium in dark for 30 min. Thereafter, the suspension was placed under 500 W xenon lamp for photocatalytic degradation reactions. Subse-quently, an appropriate amount of the suspension was taken out and followed by the centrifugation. The centrifuged transparent solution was used for the analysis of dye degradation using UV–visible spectropho-tometer (SHIMADZU, UV-1650) at every 10 min interval. The photo-catalytic efficiencies of the materials were also measured up to five consecutive cycles and every time we have cleaned the photo-catalysts properly using deionized water. We have recovered the NMO nano-rods after photocatalysis via batch experiments. Note that we have cleaned the recovered NMO nanorods properly using deionized water to remove the organic moieties and then dried prior to use for further studies. Moreover, we have characterized the recovered samples again using the XRD and electron microscopic techniques to confirm the phase purity and surface morphology of NMO nanorods. The electrochemical measurements for glucose sensing and asymmetric supercapacitors were investigated on three-electrode cell containing working (glassy carbon for glucose sensing and carbon paper for specific capacitance), counter (Pt wire) and reference (Ag/AgCl) electrodes in alkaline medium (0.1 M KOH for glucose sensing and 5 M KOH for supercapacitors). Cyclic voltammetry (CV), linear sweep voltammetry (LSV), chro-noamperometry (CA) and charge-discharge (CD) experiments were conducted on a computer-controlled electrochemical workstation (CHI 760E). The working electrode for glucose sensing has been prepared using 5 mg of powder sample with isopropanol (1 mL) and nafion (0.2 mL). A drop of this homogenous solution was put on the surface of glassy carbon electrode and used as the working electrode for the electro-chemical glucose sensing measurements. A different way has been used to prepare the electrodes for the investigation of super-capacitive behavior. We have prepared slurry for the supercapacitors containing powder sample (75%), carbon black (15%), and PVDF (10%). This slurry was pasted on the carbon paper (1 mg/cm2) and then dried. This elec-trode has been used for the performance of the supercapacitors at room temperature.

3. Results and discussion

The prepared NMO nanorods were initially characterized by Raman

and FTIR spectroscopic techniques. Fig. 1a shows the Raman spectrum of NMO nanorods. The characteristics Raman bands of NMO nanorods were appeared at 365, 705, 816, 915 and 962 cm− 1. A weak Raman bands at ~365 cm− 1 and ~ 705 cm-1 can be ascribed to the Mo-O-Mo and Ni-O-Mo of NiMoO4 [41]. The Raman bands at ~816, ~915 and ~ 962 cm− 1 can be attributed to Mo––O vibrations of NiMoO4 [41]. FTIR study of NMO nanorods is shown in Fig. 1b. Pure phase of α-NiMoO4 shows strong FTIR bands below 1000 cm− 1 corresponding to the Mo–O and Ni–O bonds, which is consistent with previous reports [42,43]. The FTIR bands at 600–700 and 930–970 cm− 1 can be attrib-uted to the stretching vibration of MoO6 octahedra of α-NiMoO4. The band appears at 804 cm− 1 can be ascribed to the stretching vibrations of Mo–O–Mo of α-NiMoO4. The FTIR band at 400–450 cm− 1 belongs to the stretching vibration of NiO6 polyhedra of α-NiMoO4.

Optical properties were also studied in order to estimate the band gap energy of NMO nanorods. The absorbance spectrum of NMO nanorods shows a peak at ~402 nm (Fig. 2a). The band gap energy of NMO nanorods was found to be 2.25 eV (Fig. 2b), which is calculated through Tauc’s model [44]. The band gap energy of NiMO4 was previ-ously reported in the range from 2.2–2.3 eV [22,45,46]. Therefore, our result is in good agreement with the reported band gap energy of α-NiMoO4. Photocatalytic activity of NMO nanorods was investigated via degradation of MB dye under visible light. Photocatalytic activity

400 600 800 1000 1200

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816

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R

Raman Shift (1/cm)

365

705

915

a

b

400 800 1200 1600 2000 2400 2800

d = 400 - 450 cm-1c = 600 - 700 cm-1b = 804 cm-1

d

b

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a = 930 - 970 cm-1

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Fig. 1. (a) Raman and (b) FTIR spectroscopic studies of the prepared NMO nanorods.

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was monitored by the UV–visible absorption spectra at λmax of 663.5 nm (Fig. 3a). We observed that the optical absorption band intensity of 663.5 nm decreases with increment of the time and decolorized. Photo- degradation of MB dye solutions reached to ~90% in 160 min in

presence of NMO nanorods (fig. 3b). The degradation of dye was calculated with the help of given equation i.e. [(Concentration with time /Initial concentration) × 100%]. Previously, Yang et al. have reported that MB dye was degraded of ~80% in 120 min and our results are consistent with this report [22]. The mechanism of photocatalytic degradation of MB dye on the surface of the photo-catalyst has been reported elsewhere [6,47]. Photocatalytic mechanism was explained on the basis of the formation of hydroxyl (OH•) and super-oxide free rad-icals (O2− •) from electron - hole (e– - h+) pairs followed by the attack to MB dye for fragmentations/intermediates. The intermediates were further degraded into inorganic minerals at long irradiation time as also reported [48]. The photo-degradation of MB dye and mineralization can be occurred to CO2, H2O, and inorganic ions like sulfate and ammonium ions or other volatile compounds with irradiation time of >1000 min [48]. A linear plot of ln(Initial concentration/ Concentration with time) versus time for degradation of dye is shown in Fig. 3c. From the linear fitted curve, we observed that the photo-chemical reaction follow first order kinetic with the rate constant of 0.013 min− 1 and R2-value of 0.9966. This is an important point to check the stability of the photo- catalysts, which is quite significant for the commercial purpose. Fig. 3d shows the cyclic stability plots of NMO nanorods during photo- degradation reactions of dye for five consecutive cycles under visible light irradiation. Our results show that the NMO nanorods as the photo- catalysts are significantly stable with marginal reduction in efficiency up to 5th cycle. Present results reveal the stability of the materials as stable photo-catalysts during photochemical performance. Moreover, we have recovered the photo-catalysts via batch process for further studies.

We have characterized the recovered NMO nanorods from the XRD

Fig. 2. (a) Absorption spectrum and (b) band gap energy plot of NMO nanorods.

Fig. 3. (a) absorption spectra, (b) percentage degradation, (c) linear plots, and (d) recycled photocatalytic activity of NMO nanorods in degradation of MB dye.

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and electron microscopic techniques prior to use as electrode materials in electrochemical reactions. The stability of the materials was tested by XRD and electron microscopic studies. Fig. 4a and b show the XRD patterns of NMO nanorods before and after photocatalytic studies respectively. The XRD patterns of NMO nanorods can be indexed in monoclinic phase of α-NiMoO4 and also match with the JCPDS file of α-NiMoO4 (# 33–0948). A careful analysis of XRD patterns of the ma-terials after photo-catalysis shows the appearance of an additional peak at two theta of ~28o, which could be due the degradation of material in small extent during photo-catalytic measurements. The additional peak could be analyzed as an impurity phase of α-MoO3. Fig. 4c and d show the SEM micrographs of NMO nanorods before and after photo-catalysis respectively. TEM and high resolution TEM images of NMO nanorods after photo-catalysis are shown in Fig. 4e and f. The average diameter of the nanorods is found to be 50 ± 5 nm from the electron microscopic studies, which also matches with the crystallite size obtained from the XRD line broadening studies. HRTEM study of NMO nanorods shows the lattice fringes with the d-spacing values of ~3.089 Å. The resulting d- spacing value of NMO nanorods is in well agreement with 〈220〉 basal plane of monoclinic phase of α- NiMoO4.

Electrochemical activity of re-covered NMO nanorods was evaluated for glucose oxidation in 0.1 M KOH solution. Fig. 5a shows CV curves NMO nanorods in 0.1 M KOH in the potential range of − 0.2 to 0.7 V versus Ag/AgCl using various range of glucose concentration from 0 to 10 μM at 25 mV/s (Fig. 5a). This is noteworthy that the appearance of a pair of oxidation-reduction peaks in CV curves indicates the redox behavior with respect to Ni component of NMO nanorods. A faradaic redox behavior of NMO nanorods can be due to Ni(II)↔Ni(III) + e− . The electro-catalytic current response in anodic region for glucose oxidation increases with glucose concentration. The hysteresis behavior and anodic current response in CV curves changes with the concentration of glucose due to electron transfer between glucose and electrodes during electrochemical glucose oxidation reactions. The anodic peak has also been shifted slightly to higher value by increasing glucose

concentration. These electro-catalytic effects can be ascribed to oxida-tion of glucose to gluconolactone. Fig. 5b shows a plot between anodic peak current densities versus glucose concentration at 25 mV/s, which display a linear response with the correlation coefficient (i.e. R2) of 0.9911. Fig. 5c shows LSV plots of NMO electro-catalysts for glucose oxidation reactions at 10 μM concentration using various scan rates of 10, 25 and 50 mV/s versus Ag/AgCl in 0.1 M KOH solution. The resulting LSV data strongly support to the CV results. Chrono- amper-ometry (CA) experiments were also carried to confirm the performance of glucose sensing at the fixed potential of 0.7 V for 600 s in 0.1 M KOH versus Ag/AgCl (Fig. 5d). From the CA data, we clearly observe the stable current densities with the change in the oxidation current den-sities of NMO electro-catalysts using 0.0 μM and 10.0 μM glucose con-centration for 600 s. Therefore, these are promising candidate for glucose sensors and the recovered NMO nanorods also exhibit efficient glucose sensing performance. Fig. 6 shows the comparative electro-chemical glucose sensing study of NMO nanorods (before and after photo-catalysis) through the potential window of − 0.2 to 0.7 V using 10 μM glucose concentration in 0.1 M KOH electrolyte solution at 25 mV/s. The anodic current density of fresh NMO nanorods is found to be 18 mA/ cm2, which is better than that of the current density obtained by the used NMO nanorods (i.e. 15 mA/cm2) as expected. Our results are also in good agreement with previous reports of non-enzymatic glucose sensing of NiMoO4 electro-catalysts [28,29].

The recovered NMO nanorods were also used to evaluate the elec-trochemical performance for asymmetric supercapacitor. CV measure-ments of recovered NMO nanorods were carried out in the potential range from 0 to 0.5 V versus Ag/AgCl at the various scan rates (2–100 mV/s) in 5 M KOH solution to estimate the value of specific capacitance in F/g (Fig. 7a). CV experiments reveal the oxidation-reduction peaks in anodic region, which correspond to the Ni (II) ↔ Ni (III), demonstrating the symmetric super-capacitive behavior of the electrode material.

We have observed that the oxidation and reduction peaks were shifted to the higher potential side and lower potential side,

Fig. 4. XRD patterns of NMO nanorods (a) before and (b) after photo-catalysis. (c) SEM image of NMO nanorods before photo-catalysis. (d) SEM, (e) TEM and (f) HRTEM images of NMO nanorods after photo-catalysis.

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respectively, with the scan rates. The change in the redox peak positions could be due to the increment in resistance at high scan rate. The recovered NMO nanorods show high area under the CV curves, which

demonstrates to the high value to the specific capacitance. The specific capacitance of recovered NMO nanorods was found to be

~660 F/g at the scan rate of 2 mV/s using 5 M KOH electrolyte solution. Previously, the specific capacitances of nanocrystalline NiMoO4 parti-cles were reported of ~178 F/g at 10 mV/s in 2 M KOH [49] and ~ 600 F/g at 2 mV/s in 2 M KOH [50]. The specific capacitance of freshly prepared NiMoO4 nanorods was reported of ~730 F/g at 5 mV/s in 3 M KOH [51]. The NiMoO4-CNTs nanocomposites supported by NF- substrate have also been reported as efficient electrode materials for supercapacitors (~2046 F/g at 2 mV/s in 2 M KOH) [50]. Bo Yu et al. have reported the NiMoO4/CoMoO4 nanocomposites wrapped by Ni-Co- S with the specific capacitance of ~778 F/g at 5 mV/s in 1 M KOH [39]. The CoO/NiMoO4 nanohybrid exhibited high specific capacitance of ~2332 F/g at 2 mA/cm2 in 6 M KOH [52]. Fig. 7b shows the charge- discharge (CD) curves of recovered NMO nanorods in the potential range from 0 to 400 mV versus Ag/AgCl at various current densities from 2 to 20 A/g in 5 M KOH. The specific capacitance of the recovered NMO nanorods was also calculated from the CD curves. The specific capacitance was found to be ~590 F/g at the current density of 2 A/g in 5 M KOH. From charge discharge studies, the specific capacitance of NiMoO4 nanorods was previously reported of ~70 F/g at 2 A/g in 3 M KOH [51]. The mesoporous and nanoporous NiMoO4 materials were reported as high performance supercapacitors with the specific capaci-tances of ~1650 and ~ 2350 F/g, respectively, at 2 A/g in 3 M KOH [53,54]. The hollow nanorods of NiMoO4 were exposed the specific capacitance of ~1100 F/g at 1 A/g in 2 M KOH [37]. The specific

Fig. 5. (a) CV curves of recovered NMO nanorods at various concentrations of glucose in 0.1 M KOH. (b) Linear plot of glucose concentrations versus anodic current density. (c) LSV plots of NMO nanorods at various scan rates using 10 μM of glucose in 0.1 M KOH. (d) CA studies of NMO nanorods in presence and absence of glucose at 0.7 V for 600 s in 0.1 M KOH for electrochemical glucose sensing.

Fig. 6. CV curves of NMO nanorods (before and after photo-catalysis) using 10 μM glucose concentration in 0.1 M KOH at 25 mV/s for electrochemical glucose sensing.

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capacitance of NiMoO4@ g-C3N4 nanocomposites was reported of ~300 F/g at 1A/g in 1 M KOH [55]. NiMoO4@rGO nanocomposites as pseudo- capacitors were also reported and exhibited proficient specific capaci-tances of ~1200 and ~ 592 F/g at 1 and 20 A/g, respectively, in 2 M KOH [56]. Fig. 7c shows long range CV curves for the stability check from − 0.2 to 1.0 V versus Ag/AgCl at 25 mV/s in 5 M KOH for 500 cycles. We observed that the current densities and the area under the CV curves were quite stable up to 300 cycles at 25 mV/s, while the current densities and the area under the CV curves decreased after 300 cycles. Therefore, we have again measured the electrochemical performance of NMO nanorods for supercapacitors after 500 cycles. CV experiments after 500 cycles were also conducted using the same potential range (0 to 0.5 V versus Ag/AgCl) and scan rates (2–100 mV/s) as shown in Fig. 7d. The specific capacitance was found to be 210 F/g at 2 mV/s after 500 cycles. Fig. 7e shows the CA studies of the electrodes for the stability check at various potentials after 500 cycles. CA measurements (i vs t) were analyzed at the fixed potentials of 650, 700 and 750 mV for 300 s in 5 M KOH, which confirm the stable behavior of the electrode materials with very small energy loss of 10 mV in 5 M KOH at 750 mV for 300 s as shown in Fig. 7f. Therefore, present work has been focused on the use of recycled NMO nanorods for multi-functional catalytic activities, which could be useful for commercialization.

4. Conclusion

NMO nanorods were used as the visible light photo-catalysts in de- colorization of the dyes. Thereafter, the photo-catalysts were recov-ered via batch process and subsequently used in electrochemical glucose sensing and supercapacitors. The recovered NMO nanorods show sig-nificant performance in electrochemical glucose sensing application in alkaline medium. The recovered NMO nanorods were also used as the promising electrode materials for supercapacitors with the specific capacitance of ~660 F/g at 2 mV/s in 5 M KOH. The stability of recovered materials was also tested by CV and CA studies. This is

noteworthy that NiMoO4 nanorods could be considered as recyclable cost effective catalysts in multipurpose technologies for commercialization.

Declaration of Competing Interest

We declare that all the authors are aware of the submission of the manuscript in Materials Characterization and they have no conflict of interest.

Acknowledgements

The authors extend their appreciation to the Deputyship for Research & Innovation, “Ministry of Education” in Saudi Arabia for funding this research work through the project number IFKSURG-1439-087.

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Fig. 7. (a) CV curves of recovered NMO nanorods from 0 to 0.5 V at various scan rates (2–100 mV/s) in 5 M KOH. (b) CD curves of NMO nanorods from 0 to 400 mV at various current densities (2 to 20 A/g). (c) CV curves for stability test from − 0.2 to 1.0 V versus Ag/AgCl at 25 mV/s in 5 M KOH for 500 cycles. (d) CV mea-surements after 500 cycles at various scan rates (2–100 mV/s) in 5 M KOH. (e) CA plots after 500 cycles at various fixed potentials in 5 M KOH. (f) Potential energy loss during CA measurement at 750 mV with time.

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