Multifunctionality of Partially Reduced Graphene OxideâCrVO4

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Multifunctionality of Partially Reduced Graphene Oxide−CrVO4 Nanocomposite: Electrochemical and Photocatalytic Studies with Theoretical Insight from Density Functional Theory Ganesh Bera,† Aradhya Mishra,† Priyanath Mal,† Amirthapandian Sankarakumar,‡ Pintu Sen,§ Abhijeet Gangan,∥ Brahmananda Chakraborty,∥ V. R. Reddy,⊥ Pradip Das,† and G. R. Turpu*,† †

Department of Pure and Applied Physics, Guru Ghasidas Vishwavidyalaya, Bilaspur 495009, India Materials Science Group, Indira Ghandhi Centre for Atomic Research, HBNI, Kalpakkam 603102 India § Variable Energy Cyclotron Center, 1/AF Bidhannagar, Kolkatta 700064, India ∥ High Pressure and Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400008, India ⊥ UGC-DAE CSR, University Campus, Khandwa Road, Indore 452001, India

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ABSTRACT: The CrVO4 and reduced graphene oxide (rGO)−CrVO4 composite has been synthesized through solid-state reaction and modified Hummer method and ultrasonication-assisted solid-state reaction method, respectively. The structural, compositional, and morphological features were studied by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy techniques. The Raman spectroscopy shows the indicative GO, and CrVO4 peaks, respectively, and the coexistence of these peaks in composite material. The photocatalytic activity of rGO−CrVO4 composite evaluated by photocatalytic degradation of methylene blue dye under UV-light irradiation shows improved ability for composite as compared to the parent compound CrVO4. The rate constant evaluated for rGO−CrVO4 is 6.7 × 10−2 min−1. Electrochemical characterization through cyclic voltammetry, galvanostatic charging and discharging, and impedance analysis was carried out. The rGO−CrVO4 composite exhibits a higher specific capacitance, 102.66 F/g @ current density of 0.5 mA/cm2, as compared to the parent CrVO4, 30.87 F/g @ current density of 0.5 mA/cm2. We have also performed extensive density functional theory simulations to qualitatively support our experimental findings by presenting the quantum capacitance and providing the insight from electronic density of states for enhanced capacitance for rGO−CrVO4 nanocomposite. Presence of additional C 2p states from rGO results enhanced the density of states which is responsible for improved supercapacitance performance of rGO−CrVO4 compared to bare CrVO4. These encouraging results illustrate the exciting potential for-high performance applications for GO composites with multifunctionality for multiple applications.

1. INTRODUCTION Multifunctional materials with a potential to address the rising global challenges, such as energy crisis, global warming, and environmental pollution, are being researched in recent years to combat these issues successfully. Graphene and its related materials can be candidate materials for various application areas , such as energy storage, CO2 capture, and photocatalytic cleaning of environmental pollutants because of its excellent electrical conductivity, larger surface area, and superior chemical stability. To further augment strength to the studies, composite materials having multifunctional nature are leading the material research from the front to find the solutions to the abovementioned issues. The global energy paradigm is rapidly transforming from nonconventional energy sources to sustainable and conventional energy sources. In this perceptive, it is necessary to find eco-friendly, low cost, and high-performance energy-storage devices. Electrochemical capacitors, also called super capacitors, efficiently fill the energy gap between © XXXX American Chemical Society

conventional capacitors and batteries with their intriguing properties such as high power capability, long cycle life compared to batteries, fast delivery rate, and faster charge/ discharge process in electric devices and electric vehicles. Supercapacitors store energy via two mechanisms: pseudocapacitance and electric double-layer capacitance (EDLC). In EDLC, charges are accumulated at the electrode−electrolyte interface. Carbonaceous materials, such as graphene and its oxides, carbon nanotubes, and activated carbons, are promising candidates because of their large surface area and superior chemical stability. Because of the limited capacitance of graphene, chemically modified graphene and graphene oxide (GO)-based composite materials with various functional Received: June 2, 2018 Revised: August 28, 2018

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DOI: 10.1021/acs.jpcc.8b05291 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

proved to be a powerful candidate for micro-supercapacitor33 and photocatalysis applications.34 Additively, elemental vanadium can form a range of compounds in combination with various elements because of its multiple oxidation states (V2+, V3+, V4+, and V5+). Hence, vanadium-based oxides possess disparate structure combined with variations in physical and chemical properties.35,36 Transition metal vanadates are widely studied for electrochemical,37 catalysis,38 gas sensing,39 and photoluminescence40 applications. Orthovanadates such as zinc vanadate,41 sodium vanadate,42 silver vanadate,43 niobiumdoped bismuth vanadate,44 manganese vanadate,45 lithium vanadate,46 copper vanadate,47 bismuth vanadate,48 RVO4 where (R = La, Ca, Nd, Sm, Eu, and Gd),49 lanthanum vanadate,50 and BiVO4/FeVO4 heterojunction51 have shown great electrochemical properties. These wide range of compounds are scarcely studied as composites with carbonaceous materials, and this leaves huge potential to develop new multifunctional materials for future applications. CrVO4, having orthorhombic structure with Cmcm space group, shows interesting photocatalytic properties and is least studied among the orthovanadate family. Here, we show the multifunctional behavior of CrVO4 and its composite with partially reduced GO (rGO−CrVO4) through electrochemical and photocatalysis experiments and compare the results. We have also presented the theoretical insight for enhanced capacitance of the hybrid rGO−CrVO4 compared to parent compound CrVO4 through state-of-the-art DFT simulations.

materials provide an alternative for supercapacitor applications.1−8 Applications of metal oxides−GO composites for water treatment from organic pollutants, heavy metals, and microorganisms were reviewed thoroughly by Upadhyay et al. owing to the importance of these materials in environmental protection;9 another review by Khan et al. on GO-based metal oxide composites also elucidates the importance of GO-based composites for various applications.10 Graphene, GO, and reduced GO (rGO) conjugation with metal oxides have been studied widely through theoretical and experimental methods to explore materials for better photocatalytic and energy applications. Role of oxygen percentage was studied through density functional theory (DFT) in g-C3N4highly rGO for photo catalytic applications and shown that the DOS of GO is different for different oxygen contents in the system.11 Also, studies related to interfacial interaction of semiconductor and GO and its effect on the end properties were done thoroughly by DFT by Xu et al.12 Several experimental tests of these materials were also done by using composite materials including, that is, rGO−Co3O4,13 NiCo−OH/rGO,14 Fe3O4/rGO,15 RuO2− rGO, 16 NiO 2 −rGO, 17 MnWO 4 /rGO, 18 AlVO 4 /rGO, 19 Fe2O3−3D graphene networks,20 intercalated GO21 as an alternative for super capacitors because of their extra pseudocapacitance compared to the pristine GO. Environmental protection is a major issue around the world; the waste from industries are dumped into the water sources containing carcinogenic pollutants, organic pollutants, and dyes which can harm the living and also causes the problem of water scarcity and quality. Photocatalysis is a technique which can be used to degrade the above-mentioned pollutants and considered as a promising approach because of its simplicity, low cost, and capability for reusing photocatalysts. Photocatalyst is a material which can absorb light, produce electron−hole pairs that can enable chemical transformation of reactant, and change its chemical composition. There are two types of photocatalytic reactions, homogeneous and heterogeneous photocatalysis. The most significant properties of a photocatalytic system are desired band gap, suitable morphology, high surface area, stability, and reusability. In this process, photocatalyst is activated by UV and/ or visible light (depending on the energy band gap of material) by exciting a photoelectron, which goes from valance band (VB) to the conduction band (CB), forming an electron−hole pair. The photogenerated electron hole pair is able to reduce and/or oxidize the compound which is there on the photocatalyst surface. The photocatalytic process follows two mechanisms: (i) formation of OH radical by OH anion and (ii) formation of O2 radical by reduction of O2. Metal oxides and semiconductors having the above-mentioned characteristics are prime candidates for photocatalysis applications.22−28 The only problem occurs in this type of reaction is the rapid recombination of electron and hole pairs, which affects the complete degradation of pollutants. Carbon-based materials have been proved as promising candidate materials for the prevention of recombination process. GO contains oxygenated functional groups such as epoxy, hydroxyl, and carboxylic acid on hexagonal carbon lattice. On UV−visible light irradiation, these functional groups form free radicals which further get transferred to the other oxygenated functional groups and prevent recombination of electron hole pair by generating OH radical.29 Recently, GObased composites including Fe2O3−GO,30 Bi2O3 rods−rGO,31 and TiO2−GO32 have shown capability of effective photodegradation of organic pollutants. Carbon-based materials

2. EXPERIMENTAL METHODS CrVO4 was synthesized by a standard solid state-reaction method with stoichiometric ratios of V2O5 and CrO3, with purities greater than 99%. Materials were ground before treating them for calcination at 773 K for 10 h. The resultant black powder was ground and pelletized before sintering at 923 K for 12 h in a muffle furnace. GO was synthesized by standard modified Hummers method.52 Graphite (2 g) powder and 1 g of NaNO3 were taken in a container, and the container was kept on an ice bath having temperature 10 wC. Concentrated H2SO4 (96 mL)was added into the mixture and stirred well. KMnO4 (6 g) was added into the solution gradually, and stirring continued for 2 h. Distilled water (150 mL) was added into the solution, and the temperature of the mixture started increasing rapidly. To maintain the temperature of the solution low, it is again kept on the ice bath. Additional 240 mL of water was added to make the solution dilute and kept idle. H2O2 (30%; 5 mL) was added to the solution and mechanically stirred for 2 h. Finally, the solution was filtered and washed with 250 mL of 10% HCl, 160 mL of distilled water, and 90 mL of ethanol, respectively, and transferred into the oven for drying for 12 h at 60 °C. To get the composite of the CrVO4 and GO, 0.05 g of GO powder was dispersed in 20 mL of distilled water and ultrasonicated for 1 h 15 min to get exfoliated GO flakes. CrVO4 (0.066 g) was added into the solution and again ultrasonicated for 1 h. The solution was then filtered and dried in a furnace at 90 °C for 6 h. The obtained samples were characterized through X-ray diffraction (XRD) studies for structural identification using 9 kW Rigaku SmartLab X-ray diffractometer in a 2θ range of 10°−80° with Cu Kα radiation having a wavelength of 0.15406 nm at room temperature. Raman spectroscopic studies were carried out at room temperature using micro-Raman spectrometer of Technös Japan, with 532 nm diode laser illumination. Scanning electron microscopy (SEM) imaging was recorded using Carl Zeiss EVO B


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The Journal of Physical Chemistry C 10 SEM system. The crystal orientation, grain sizes, shape, and structure were further confirmed by using high-resolution transmission electron microscopy (HRTEM; Zeiss LIBRA 200 FE) operated at 200 keV. The UV−visible spectroscopic studies were carried out by using PerkinElmer, LAMBDA 950, UV− vis−NIR spectrophotometer under diffuse reflectance mode. The photocatalytic activities of the as-prepared polycrystalline CrVO4 and rGO−CrVO4 were evaluated by the photodegradation of methylene blue (MB) dye under visible light irradiation. Prior to irradiation, the suspension was stirred for 10 min in visible light (40 W tube light) to achieve an equilibrium mixture of 30% MB solution containing 12 mg of catalyst for rGO−CrVO4 and 10 mg of catalyst for CrVO4. Then, the photodegradation of MB dye was monitored and analyzed by UV−vis absorption spectroscopy at different time intervals. In the electrochemical study, electrodes for supercapacitor were prepared through the following procedure: 85 wt % electroactive materials (i.e., CrVO4 and rGO−CrVO4) were mixed with 10 wt % acetylene black and 5 wt % polytetrafluoroethylene to form a thick paste. The paste was then pressed into a thin sheet of 100 m thickness. Finally, the sheet was compressed on a stainless steel mesh having the surface area around 1 cm2. The prepared electrodes were dried at 60 °C for 6 h under vacuum. The total weight of the active material in the electrode is usually 5 mg. Electrochemical performances of the samples were observed using cyclic voltammetry (CV) measurement (AUTOLAB-30 potentiostat/galvanostat). A platinum electrode and a saturated Ag/AgCl electrode were used as counter and reference electrodes respectively. All of the CVs were measured between −1.0 and 1.0 V (i.e., operating window of 0.2 V) with respect to reference electrode at different scan rates (2− 20 mV/s). Galvanostatic charge−discharge cycling and electrochemical impedance studies were performed with two-electrode system having identical electrodes made of the same active electrode materials (i.e., type-I symmetric supercapacitor). Constant current density ranging from 0.5 to 10 mA/cm2 has been employed for charging/discharging the cell in the voltage range from −1.0 to 1.0 V. The discharge capacitance (C) is estimated from the slope (dv/dt) of the linear portion of the discharge curve using the expression53 C = I /(dv /dt )

(1)

Cs = 2C /m

(2)

Figure 1. (a) XRD patterns and (b) Raman spectra of CrVO4 (blue line), rGO (red line), and rGO−CrVO4 composite (black line), respectively.

CrVO4, indicating the formation of rGO−CrVO4 composite successfully. It is also observed that the peak corresponding to (001) plane of GO shifts toward the higher angles indicating the reduction of the GO by adding CrVO4 to it. It is evident that the materials prepared are pure and are in single-phase from the XRD studies. Figure 1b shows the Raman spectra of GO, CrVO4, and rGO−CrVO4 composite. The main features in the Raman spectra of graphitic carbon-based materials are the G and D peaks and their overtones. The first-order G and D peaks, both arising from the vibrations of sp2 carbon, appear around 1570 and 1354 cm−1, respectively.56 The G peak corresponds to the optical E42g phonons at the Brillouin zone center, resulting from the bond stretching of sp2 carbon pairs in both rings and chains. The D peak represents the breathing mode of aromatic rings arising because of the defect in the sample. The D-peak intensity is therefore often used as a measure for the degree of disorder. The Raman spectra of GO shows the G and D bands at 1583.27 and 1345 cm−1, respectively, with the intensity ratio between D and G peaks, ID/IG = 1.04. CrVO4 having orthorhombic crystal structure with Cmcm space group has four symmetry inequivalent atoms in the unit cell occupying 4a, 4c, 8f, and 8g positions. The group theory analysis predicts the following irreducible representation at Γ point: Γ = 5Ag + 4B1g + 6B1u + 3Au + 2B2g + 7B2u + 4B3g + 5B3u; out of these 36 phonon modes, there exist three acoustic modes (B1u, B2u, and B3u), three silent modes (Au), 15 infrared (IR) modes (5B1u, 6B2u, and 4B3u), and 15 Raman active modes (5Ag, 4B1g, 2B2g, and 4B3g). Raman active vibrational modes can be classified as internal and external modes of the VO4 units.57 The modes at high frequency (900 cm−1) are internal modes of VO4 tetrahedra because of symmetric and asymmetric stretching and bending. The modes at lower wave number are due to pre-translational (T) and modes in between these two are due to pure rotational (R) modes. The Raman spectra of composite material (rGO− CrVO4) show all modes as observed in the pristine compounds with a slight shift toward the lower wave numbers. The intensity ratio of D and G peaks of rGO in the composite changes to 1.09−1.04, indicating possible partial reduction of GO into rGO58 because of the formation of bonds on the rGO sheet with Cr/V ions. The visible light absorbance spectra are being studied using diffuse reflectance method. Figure 2a,b shows the diffuse reflectance spectra (DRS) of CrVO4 and rGO−CrVO4 powered sample. The absorbance spectrum of CrVO4 shows broad absorbance with multiple absorption bands (around 380, 490, and above 600 nm) in consonance with the literature.55 The composite rGO−CrVO4 shows similar broad absorption spectra with multiple absorption bands. The orthorhombic CrVO4 consists of tetrahedra VO4 groups connected by CrO 6

where m is the active mass of the single electrode and (Cs) represents specific capacitance of the electrode. Electrochemical impedance spectra (EIS) were taken at an open circuit potential over the frequency range from 10 kHz to 10 mHz with a potential amplitude of 5 mV. All of the electrochemical experiments (i.e., CV, charge discharge, EIS) were carried out in an electrolyte containing 1 M LiClO4 in acetonitrile.

3. EXPERIMENTAL RESULTS AND DISCUSSION Figure 1a shows the XRD pattern of GO, CrVO4, and rGO− CrVO4 composite. The diffraction peak indicating (001) plane of GO corresponds to the interlayer space (d spacing) of 0.75 nm, which is larger than the d spacing of natural graphite (0.34 nm).54 The XRD pattern of CrVO4 shows pure single-phase formation of the compound and is indexed to the orthorhombic structure having Cmcm space group. The lattice parameters evaluated by Rietveld analysis for CrVO4 are a = 5.5818 Å, b = 8.2390 Å, and c = 5.9953 Å.55 The prepared composite of rGO and CrVO4 shows the peaks corresponding to both rGO and C


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the CrVO4 particles on the exfoliated flakes of few layers of graphene sheets with almost minimal agglomeration of the particles. This increases the surface area of the materials which is a significant property required for both investigated functionalities in these materials. Figure 4a shows the TEM image of CrVO4 where the uniform particles with an average particle size of 25 nm are clearly visible. Whereas Figure 4b shows the exfoliated rGO showing paperlike flakes. Very simple ultrasonication-assisted method resulting in these thin flakes makes it an attractive method to be followed for composite preparation. The image of rGO−CrVO4 composite shows the elegance of the exfoliation and uniform distribution of CrVO4 on very thin sheets of rGO. The HRTEM images of rGO, CrVO4, and composite are shown in Figure 4d−f. HRTEM images of CrVO4 and rGO clearly revealing the crystal lattice with an interplanar spacing of 0.476 and 0.414 nm, respectively, which are, respectively, assigned to (110) and (002) planes of CrVO4 and rGO. HRTEM image of rGO−CrVO4 composite shows a clear interface between rGO and polycrystalline CrVO4, with a lattice spacing of 0.44, 0.46, and 0.25 nm agrees with (002), (110), and (200) lattice planes of rGO and CrVO4, respectively. The selected area electron diffraction (SAED) pattern (insets of Figure 4d−f) of compounds shows clear bright sports for CrVO4 and composite, indicating very high crystallinity of the samples. Figure 5 representatively shows the photocatalytic degradation profiles of MB over the CrVO4 and rGO−CrVO4 composite photocatalysts. MB shows high stability under visible light in the absence of the photocatalyst, as evidenced by the aging study of the solution. The photocatalytic activities of CrVO 4 and rGO−CrVO 4 composite were investigated by following the degradation of MB dyes as a cationic pollutant model. It is clearly observed that in the presence of a photocatalyst, the absorbance decreases with irradiation time because of the photocatalytic reaction. The photocatalytic reaction pathway is believed to involve the reaction of MB with the generated OH-free radicals, resulting in N-demethylation of its auxochromic dimethylamine groups. Such demethylization depredates MB into a compound with a short absorption wavelength, in addition to H2O and CO2.62 Figure 5a,b displays the intensity of absorption spectra of MB dye which is having a maximum absorbance at 668 nm and is gradually decreasing on increasing the irradiation time to 330 and 400 min for CrVO4 and rGO−CrVO4 composite, respectively. The maximal position of absorption is not altered during the removal process indicating the complete destruction of the dye pollutant into inorganic carbon, and no other intermediate organic products are formed in the degradation process.63

Figure 2. DRS of (a) CrVO4 and (b) rGO−CrVO4 composite converted to KM absorbance. The insets in (a,b) (upper right panels) illustrate Tauc plot showing optical band gap of CrVO4 and rGO− CrVO4 composite, respectively.

octahedra. This coordination of metal ions to oxygen suggests that the studied optical absorption spectra shows different types of transitions (1) charge-transfer transitions O2 → Cr3+ and (2) d−d electron transitions connected with CrO6 octahedra, which are allowed transitions in CrVO4.59 The absorption band at 380 and 480 nm in both CrVO4 and rGO−CrVO4 is associated with O2 → Cr3+ charge-transfer process, and the bands appearing at higher wavelength are associated with d−d transitions in CrO6 octahedra.60 No appreciable change is being observed in the band position because of the addition of the rGO to CrVO4. To evaluate the band gap of these materials, the spectra were analyzed with Tauc plot analysis through DRS by using the formula (αhν)2 = A(hν − Eg )

(3)

where A, α, hν, and Eg are the arbitrary constant, absorption coefficient, photon energy, and band gap energy, respectively.61 Inset of Figure 2a,b shows the Tauc plot of CrVO4 and rGO− CrVO4 composite, and the evaluated energy band gap of CrVO4 and rGO−CrVO4 are 2.44 and 2.38 eV, respectively. This variation in the energy band gap and the presence of multiple absorption bands in spectra in the visible range is a very important characteristic of CrVO4 which can be exploited for photocatalysis applications. Further addition of GO increases the potential for the use of CrVO4 for these applications under visible light irradiation. The morphology and composite nature of the compounds are further studied by SEM and transmission electron microscopy (TEM) measurements. Figure 3a−c shows the SEM image of rGO, CrVO4, and rGO−CrVO4 composite, respectively. The exfoliated nature of the partially reduced GO is clearly seen in GO sample with evidence for a flakelike structure. CrVO4 image shows particles of few tens of nanometer sizes with nearly spherical shape. The composite material shows distribution of

Figure 3. SEM images of (a) rGO and (b) CrVO4 and rGO−CrVO4 composite, respectively. D


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Figure 4. TEM images of (a) CrVO4, (b) rGO sheet, (c) rGO−CrVO4 composite and HRTEM images of (d) CrVO4, (e) GO sheet, (f) rGO−CrVO4 composite, respectively. The insets in (d−f) illustrate the SAED patterns of the same compounds.

(4)

where, C0 is the initial concentration of dye solution (mol/L), Ct is the concentration of the dye at various interval times (mol/L), t is the illumination time, and k is the reaction rate constant. The evaluated rate constants are 0.0332 and 0.0670 min−1 for CrVO4 and rGO−CrVO4 composite catalysts, respectively. It is evident that the composite has almost twofold photocatalytic activity as compared to CrVO4. This increase suggests that there is a synergistic effect between rGO and CrVO4. On the basis of the above results and discussions, we propose a mechanism for the photocatalytic activity of CrVO4 and rGO− CrVO4 composite, respectively, for the degradation of MB dye under UV−vis light irradiation. Upon absorption of light by CrVO4, an electron from its VB goes to its CB by forming a hole at the VB, but these e− and h+ so formed quickly recombine with each other because of the instability of the excited state, resulting in a low catalytic activity of CrVO4. This is avoided by the introduction of rGO, as it acts as an electron acceptor and transporter by successfully shielding the recombination of photogenerated e−−h+ pairs, as shown in Figure 7. Here, e− from the CB of CrVO4 goes to rGO sheet containing carbons, where it contacts O•2 present at the effective sites of the carbon ring and gets absorbed resulting in the formation of oxygen superoxide free radicals. On the other hand, holes from the VB of CrVO4 absorbed by the OH− group were derived by dissociation of

Figure 6. (a) Photodegradation reaction efficiency of MB solution using CrVO4 and rGO−CrVO4 composite (b) first-order kinetic linear fitting curves of MB photocatalytic degradation with same catalysts.

Figure 7. Mechanism for photodegradation reaction of MB solution using rGO−CrVO4 composite.

Figure 5. UV−visible absorbance spectra of MB dye solution with performance of (a) CrVO4 and (b) rGO−CrVO4 composite.

Figure 6a shows the degradation of initial concentration of MB dye with respect to time. The photocatalytic degradation process obeys the first-order decay kinetics as represented in Figure 6b, and the rate constants were estimated according to the following formula64 Ct = C0 exp( − kt ); or, ln C0/Ct = kt

E


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Figure 8. Cyclic voltammograms of (a) CrVO4 and (b) rGO−CrVO4 composite at different scan rates and (c) a comparison between CrVO4 and rGO−CrVO4 at 5 mV/s.

Figure 9. GCD curve of (a) CrVO4 and (b) rGO−CrVO4 composite at different current densities.

H2O molecule and form an OH• free radical, such that, the number of OH• radicals increases which enhances the degradation rate as they are having high oxidative potential for the oxidation of organic pollutants. Here, oxygen superoxide free radicals and hydroxyl free radicals oxidize MB into CO2, H2O, and other minerals. Following equations summarize the whole mechanism precisely.1

the scan rate is also observed. Notably, at the scan rate of 20 mV/ s, the behavior can be compared with other high-power SCs found in literature. Figure 8c shows a comparison between CrVO4 and rGO−CrVO4 at 5 mV/s, the three redox peaks appearing in CV of CrVO4 at −0.2768, 0.3302,, 0.7377 V in the anodic sweep and three peaks −0.7138, −0.3284, and 0.3421 V in the cathodic sweep get disappeared in rGO−CrVO 4 composite. Nearly a three times increase in capacitance is observed for composite in contrast to CrVO4 largely because of the increase in the surface area of the composite and availability of various conduction paths in rGO. Figure 9a,b shows the galvanometric charging discharging (GCD) curve of CrVO4 and rGO−CrVO4 composite at different current densities of 0.5, 1.0, 5.0, and 10.0 mA/cm2, respectively. Therefore, triangular shapes were observed for GCD curves at different current densities. The calculated specific capacitance of rGO−CrVO4 composite is 102.66 F/g for 4.45 × 10−3 g of electrode material and that of CrVO4 is 30.87 F/g at a scan rate of 0.5 mA/cm2. The discharge curve of rGO−CrVO4 is quite linear as compared to CrVO4, with a minimal IR drop visible at the start of each discharge curve. At higher scan rates, the IR drop becomes more visible. Figure 10 shows a Nyquist curve of rGO− CrVO4 composite in the frequency range of 100 KHz to 10

CrVO4 + h+ → CrVO4 (h+VB + e−CB) O2 + eCB → O•2 H 2O → H+ + OH−

OH− + h+VB → OH• MB + OH• + O•− 2 → CO2 + H 2O + ...

Figure 8a,b shows the cyclic voltammograms of CrVO4 and rGO−CrVO4 composite at different scan rates. The CrVO4 cyclic voltammogram shows various oxidative and reduction peaks, whereas in composite material, these peaks almost disappeared. The composite CV shows a nearly rectangular shape, which are typical for an electrical double-layer capacitive behavior. A gradual increase in the current with the increase in F


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using a Monkhorst and Pack72 mesh of 9 × 7 × 5 for bulk geometry and 9 × 11 × 1 k-points for (100) surface of bulk for supercapacitor study. The convergence criterion of 0.01 eV/Å for Hellmann−Feynman forces and 10−6 eV for total energy was considered. To account for weak van der Waals forces between CrVO4 and GO, we have considered Grimme DFT-D273 dispersion correction which uses a pairwise force field for describing the van der Waals interactions. 4.2. Structure and Electronic Properties of Bulk CrVO4. DFT-optimized structure of bulk CrVO4 is shown in Figure 11b with VO4 and CrO6 polyhedrons. V has a tetrahedral coordination, whereas Cr has an octahedral co-ordination, as displayed in Figure 11b. Computed lattice parameters using gradient-corrected approximation (GGA) as exchange correlation functions are a = 5.54 Å, b = 8.29 Å, and c = 6.08 Å and match reasonably well with the GGA-computed literature values of a = 5.32 Å, b = 7.96 Å, and c = 6.09 Å71 Our theoretical lattice parameters are also in good agreement with our experimental lattice parameters of a = 5.58 Å, b = 8.23 Å, and c = 6.00 Å obtained from XRD analysis and the experimental lattice parameters of a = 5.23 Å, b = 7.78 Å, and c = 6.33 Å given in the literature74 Computed Cr−O bond lengths are 1.94 and 2.02 Å in two different directions and match nicely with the corresponding experimental bond length of 1.94 and 2.08 Å.75 Similarly, computed V−O bond lengths are 1.64 and 1.75 Å in two different directions and agree reasonably well with the corresponding experimental bond length of 1.52 and 1.55 Å. Figure 11a displays the total DOS (TDOS) for bulk CrVO4. The computed band gap using GGA + U method (U = 4.5 eV for d orbital of Cr and V) comes out to be 2.35 eV, matching nicely with our experimental band gap of 2.44 eV and reference experimental value of 2.51 eV. 4.3. Surface Properties of CrVO4 and rGO−CrVO4 Electrodes. From the optimized structure of bulk CrVO4, we have constructed (100) surface and allowed it to relax. We constructed rGO−CrVO4 surface by adding GO layer around 2 above (100) surface of CrVO4. After the relaxation, the interface distance becomes 3.5 which indicates that the interaction is mainly van der Waals interactions between CrVO4 surface and GO. Figure 12 presents the DFT-optimized structure of (100) surface of CrVO4 and GO + (100) surface of CrVO4. Here, we mention that for low O coverage below 8%, the opened band gap is negligible75 and there is almost no difference in DOS between graphene and rGO. Please note that here we are interested in supercapacitance performance, and the quantum capacitance has been calculated using DOS. The experimental samples have O content less than 8%. Therefore, the DOS and quantum

Figure 10. Nyquist curve of CrVO4 (black) and rGO−CrVO4 (red) in the frequency range of 100 KHz to 10 mHz.

mHz, where there is an internal resistance Rs = 9.64 Ω in the high-frequency region; on the other hand, the Rs in CrVO4 was 15.91 Ω. The partial semicircle in the high-frequency to mid-frequency region is modeled by an interfacial charge-transfer resistance Rct = 0.854 Ω; on the other hand, the Rct of CrVO4 was 14.87 Ω. A low Rs and Rct of rGO−CrVO4, respectively, in the highfrequency region is attributed to balanced electronic and ionic conduction in composite material. Moreover, the impedance curve of composite parallel to the imaginary axis indicates the capacitive nature of rGO−CrVO4 composite. The high Rs and Rct for CrVO4 indicate high diffusion resistance and resistive electron-transfer pathways. This substantially differentiates CrVO4 and rGO−CrVO4 in turn which holds larger chargestorage capacity.65

4. THEORETICAL INSIGHT FROM DFT SIMULATIONS To support our experimental data and provide some theoretical insight for the enhanced capacitance of rGO−CrVO4 compared to pure CrVO4, we have done DFT simulations on CrVO4 and rGO−CrVO4 and presented quantum capacitance and density of states (DOS). 4.1. Computational Details. Quantum simulations have been done using the DFT-based projector augmented wave (PAW) method as implemented in the VASP code.66−69 PAW-based pseudopotentials70 have been used for Cr, V, O, and C with PW91 as the exchange−correlation functional, and the semicore p states are included for Cr and V. We have taken orthorhombic structure having Cmcm space group.71 The cut-off energy is taken to be 500 eV, and the brillouin zone is sampled

Figure 11. (a) TDOS of bulk CrVO4 and (b) its corresponding crystal structure with VO4 and CrO6 polyhedrons. V has a tetrahedral co-ordination given in red color, whereas Cr has an octahedral co-ordination given in blue. G


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Figure 12. DFT-optimized structures of (a) (100) surface of CrVO4 and (b) (100) surface of CrVO4 with GO; the red, green, purple, and blue spheres indicate Cr, V, O, and C atoms, respectively.

capacitance are almost same for graphene and rGO with O content less than 8%. That is why we have considered only graphene in the simulations. The TDOS of (100) surface of CrVO4 and GO + (100) surface of CrVO4 is displayed in Figure 13. We can notice the enhanced DOS around Fermi level for GO

Figure 14. Variation of quantum capacitance (CQ) of (100) surface of CrVO4 (blue) and (100) surface of CrVO4 with rGO (green); there is an enhancement of CQ in the hybrid CrVO4−rGO compared to bare CrVO4 in consistence with the experimental data.

1 1 1 = + CT CQ C EDL Figure 13. TDOS of (100) surface of CrVO4 (lower panel) and (100) surface of CrVO4 with rGO (upper panel); different Fermi levels are shown in magenta dotted line.

where CEDL is the EDLC which depends on electrode− electrolyte interfacial interaction. In Figure 15, we plot the partial density for Cr d orbital (lower panel), V d orbital, O p orbital, and C p orbital (upper panel) for (100) surface of CrVO4 and GO + (100) surface of CrVO4. We can notice from Figure 16 that the supercapacitance performance is mostly

+ (100) surface of CrVO4 which is responsible for improvement in supercapacitance performance of GO + (100) surface of CrVO4 compared to bare CrVO4. We compute quantum capacitance CQ from DOS using the formula76 CQ = e 2

(7)

∞

∫∞

D(E)FT(E − eφG) dx

(5)

Here, D(E) is the DOS, φG is the electrode potential, e is the electronic charge, and E is the energy, and the thermal broadening function, FT(E), is given by FT(E) = (4KBT )−1 sec h2(E /2KBT )

(6)

where KB is the Boltzmann constant and T is the temperature. Figure 14 presents the variation of quantum capacitance of (100) surface of CrVO4 (blue) and (100) surface of CrVO4 + GO with an electrode potential. It is very clear from Figure 13 that the quantum capacitance is higher for GO-supported CrVO4 compared to pure CrVO 4 which supports our experimental data. Here, we point out that in experiment, we measure total capacitance, and for low-dimensional electrode, it is expressed as44

Figure 15. Partial DOS of (100) surface of CrVO4 with rGO for Cr d orbital, V d orbital, O p orbital, and C p orbital; Fermi level is shown by dotted magenta line. H


Article


(2.35 eV) and lattice parameters for bulk CrVO4 matches nicely with our experimental data. The presence of additional C p states from GO and its hybridization with d orbital of Cr and V results enhanced DOS and higher capacitance for rGO−CrVO4 nanocomposite which corroborates experimental predictions.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Brahmananda Chakraborty: 0000-0001-9611-099X G. R. Turpu: 0000-0002-1435-4091 Notes

The authors declare no competing financial interest.

■

Figure 16. Charge density isosurface of (100) surface of CrVO4 for isovalue 0.1e; yellow color depicts the charge density iso-surface; red, green, and purple spheres indicate Cr, V, and O atoms, respectively.

ACKNOWLEDGMENTS G.R.T. acknowledges the financial support through major research project of University Grants Commission vide F. no. 43-407/2014(SR). G.R.T. and G.B. acknowledge the UGCDAE CSR, Indore and Mumbai center, for the financial support vide nos. CSR/CRS87/201415/594 and UDCSR/MUM/CD/CRSM-263/2017/1036, respectively. Authors from GGV, Bilaspur, thank UGC and DST, Government of India, New Delhi, for supporting the Department of Pure and Applied Physics through UGC SAP DRS-I and FIST Level-I programs, respectively. B.C. would like to thank Dr. A. K. Mohanty for his constant support and encouragements. B.C. would also like to thank BARCs supercomputing facility for their help. B.C. would also like to thank Dr. Lavanya M. Ramaniah for support and encouragements.

exhibited by d orbitals of V and Cr, and O p orbitals have negligible contributions. Figure 15 depicts the charge density isosurface of (100) surface of CrVO4 for an isovalue of 0.1e; yellow color displays the charge density isosurface; red, green, and purple spheres indicate Cr, V, and O atoms, respectively. From DFT simulations, we can infer that the appearance of additional C p states from GO enhances the DOS near Fermi level and improves the supercapacitance performance of GO + CrVO4 compared to pure CrVO4, which supports our experimental observations.

5. CONCLUSIONS In summary, rGO−CrVO4 composite has been synthesized through a facile ultrasonication-assisted solid-state reaction method which displays significantly improved photocatalytic and electrochemical properties for supercapacitor applications in contrast to the CrVO4. The room-temperature XRD and Raman spectroscopic measurement demonstrated the structural phase purity of the obtained compound. SEM, TEM, and HRTEM images are consistent with each other and reveal the incorporation of CrVO4 nanoparticles on the graphene flakes. The electronic band gap of the compounds is evaluated with the help of diffuse reflectance UV visible spectra, where the band gap of CrVO4 and rGO−CrVO4 were found to be 2.44 and 2.38 eV, respectively. rGO−CrVO4 shows superior ability of photodegradation of MB with a rate constant of 6.7 × 10−2 and shows good electrochemical properties having specific capacitance of 102.66 F/g for supercapacitor applications. The GCD curve of rGO−CrVO4 shows pure capacitive nature which suggests that the composites of GOs with orthovanadates are the potential candidates (materials) for the electrochemical application. DFT-predicted band gap (2.35 eV) and lattice parameters for bulk CrVO4 matches nicely with our experimental data. Appearance of additional C 2p states from rGO near Fermilevel results enhanced DOS and higher capacitance for rGO− CrVO4 nanocomposite, which corroborate experimental predictions. rGO−CrVO4 shows superior ability of photodegradation of MB with a rate constant of 6.7 × 10−2 and shows good electrochemical properties having specific capacitance of 102.66 F/g for super capacitor applications. The GCD curve of rGO− CrVO4 shows pure capacitive nature which suggests that the composites of GOs with orthovanadates are potential candidates for the electrochemical applications. DFT-predicted band gap

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