Enhancing Dye Degradation Property of MoO3 Nanoplates by Vanadium Doping

Nanomaterial based water degradation is becoming as a promising option in comparison to conventional water degradation methods. MoO3 nanoparticles have been used as a nano adsorbent for methylene blue (MB) removal from aqueous solution. Here, effect of vanadium (V) element doping in MoO3 on adsorption activity against MB was studied. 2%, 4%, 6% and 8% of V element doped MoO3 nanoparticles were synthesized using surfactant free chemical method. All the synthesized nanoparticles were well characterized through different analysis tools to study their structural, morphological, and optical properties. Stability of particles in water with respect to time was also studied by zeta potential. Adsorption activity of all the samples were carried out and 8% doped MoO3 nanoparticle was found to be most efficient. Moreover, the regeneration and reusability test of 8% doped MoO3 nanoparticle was also successfully carried out.


Experimental
Synthesis method and characterizations.-AnalyticalReagent grade chemicals were used for the synthesis of pure α-MoO 3 and V element doped MoO 3 up to 8% doping concentrations by a facile liquid state chemical reaction method 19 and labeled as MoO 3 , M98 (2%), M96(4%), M94(6%), and M92(8%) respectively.The synthesized annealed nanoparticles were examined using various analysis tools.X-ray diffraction (XRD, Bruker make), Raman spectroscopy (Renishaw make), Scanning electron microscopy & Energy dispersive X-ray analysis (SEM with EDAX, JEOL make) and Diffused reflectance spectroscopy (DRS, Shimadzu make UV3800) were used to study structural, morphological, elemental and optical properties of samples.The surface and pore size were measured using Brunauer-Emmett-Teller (BET, Quantachrome Instruments) analysis.The stability of particles in water was studied using Zeta potential (Malvern).
Adsorption measurements.-Theadsorption experiment was carried out in similar fashion as in our earlier work. 20Here adsorption activity in Methylene blue(MB) solution was carried out for V element doped MoO 3 samples along with MoO 3 sample.Removal percentage of MB, maximum adsorption capacity at equilibrium (q e ) and Gibbs free energy change for all the samples were calculated using below Eqs.1-3, respectively. 17,21(( − )/ ) × ( ) Where, C i and C f are the initial and equilibrium concentration (mg/L) of dye respectively, V is the volume of dye solution (L) and m is the mass of adsorbent (g).The Gibbs free energy change is given by the

Result and Discussion
Structural analysis.-Figure1(a) shows the XRD plot of α-MoO 3 , M98, M96, M94 and M92 nanoparticles in the range of 10°to 45°.All the XRD peaks and corresponding diffraction angles are well matched with the JCPDS card No-05-0508, that signifies to MoO 3 orthorhombic structure.Increase in vanadium doping concentration shows no shifting of peaks in MoO 3.Moreover, vanadium or vanadium oxides series peaks are also absent.This indicates well insertion of vanadium element in MoO 3 and forming stable V element doped MoO 3 solid solution.This may be due to similar ionic radius of Mo and V. 22,23 The change in doping concentrations has only shown variation in intensity of peaks which is due to increase in domain population for the corresponding planes.Lattice parameter a, b and c, average crystallize size (D = kλ/βcosӨ), induced z E-mail: jvishu1990@gmail.com;dshah@phy.svnit.ac.inSurface morphology and elemental composition analysis.-Asshown in Figs.2a-2e, the surface morphology of all the samples was studied using the SEM technique with a resolution of 1 μm.The SEM image of α-MoO 3 shows nanoplates of irregular shape (150--300 nm thickness and 1-5 μm length).The SEM images of V element doped MoO 3 evidently indicate the dissimilarity in morphology with change V element doping concentration in MoO 3 .No much variation in range of thickness was observed whereas, uneven elongation in length and width was found.SEM image shows high dispersity and uniformity of V element doped MoO 3 nanoparticles synthesized as no other morphologies were observed.Moreover the V element doping has not distorted the layered structure of MoO 3 .Figures 2a-2e 24,25 It can be seen that all the peaks observed are present in all the samples.There is no shifting of peaks, only disparity in intensity of peaks due to V element doping.A variety of stretching vibration modes are represented by the Raman shifts between 1000 and 600 cm −1 , whereas deformation vibration modes are represented between 600 and 200 cm −1 .Raman peaks at 666, 819, and 995 cm −1 correspond to the stretching vibration modes of α -MoO 3 phase. 26No peaks correlated to other phases MoO 3 were observed in all samples spectra.Thus, the Raman and XRD analysis leads to the same result.
Optical analysis.-Opticalproperties of α-MoO 3 , M98, M96, M94 and M92 were studied using diffuse reflectance spectroscopy (DRS) in the range of 300 nm to 800 nm at ambient temperature.The Kubulka -Munk (K-M) function (Eq.4) was used to calculate the bandgap of all samples. 19− Where F(R) is the Kubulka-Munk function, K is the coefficient of absorption, S is the scattering and R ∞ is the reflectance of the sample at infinite thickness.Figure 4 shows (F(R)hν) 1/2 versus hν plot of all the samples.Decrease in bandgap is been witnessed with increase in V element doping in MoO 3 .The replacement of Mo by V causes no change in the crystal structure, however it induces a mid-gap state in the electronic structure of MoO 3 .This leads to reduce in band gap with increase in V element doping in MoO 3 . 27unauer-emmett-teller analysis.-BETmethod through nitrogen adsorption and desorption was employed to study specific surface area, and pore size of all the samples.Increase in V element doping in MoO 3 , causes the increase in surface area of samples and is shown in Fig. 5.The observed pore size for samples MoO 3 , M98, M96, M94, and M92 are 18.35, 19.24, 24.75, 27.74 and 19.76 Å respectively.The high surface area and varying pore size are beneficial for adsorption applications.
Zeta potential analysis.-Thezeta potential is a vital parameter for understanding the colloidal dispersion stability.The amount of electrostatic repulsion between close, similarly charged particles is depicted by the zeta potential.Significantly small molecules and particles are given stability by a high zeta potential, which means that the solution or dispersion will resist accumulation.At low potential, the attractive forces can overcome repulsion, causing the   ECS Advances, 2023 2 042003 dispersion to break apart and flocculate.Low zeta potential (negative or positive) colloids coagulate or flocculate, whereas high zeta potential (negative or positive) colloids are electrically stable.Table I gives the range of stability behavior and zeta potential dependence of a colloidal solution. 28o study the stability of prepared samples under water, the surface charge on them was measured through zeta potential.It was observed that the samples nanoparticles show good stability and their stability increases slightly with respect to time.The measured values of zeta potential are listed in Table II.Such type of behavior is favorable for wastewater where the particles are in continues contact with water.
Adsorption study.-Adsorptionstudies were often carried out depending on variables like adsorbent loading, temperature, pH of solution, initial MB concentration, and time. 17,29In this study, the removal of MB from the aqueous solution at natural pH conditions was exclusively examined as a function of the concentration of V element doping in MoO 3 .The trial was conducted with an initial MB concentration of 10 mg l −1 at an ambient temperature during an 80-minute contact period.To understand the behavior of MB degradation with respect to time in water without adding any adsorbent, UV-visible spectra at 0 min and 80 min were taken.It is observed in Fig. 6a that there is no degradation of MB with respect to time in water.The adsorption studies MB with adsorbent sample MoO 3 , M98, M96, M94 and M92 with respect to time is shown in Figs.6b-6f respectively.It is observed from these figures that the adsorption of MB increases from 69.06% to 96.89% with increase in V element doping concentration percentage from 0 to 8 % in MoO 3 .
The removal percentage of MB at the end of 80 min for each sample was calculated using Eq. 1 and has been shown in Table III.It is evident from Fig. 7, the removal % of MB for sample M94 and M92 shows negligible change after 60 min whereas as for other sample i.e MoO 3 , M98 and M96 shows increase in removal % of MB with respected to time for the given time interval.The Table III also contains the adsorption capacity and change in Gibbs free energy for each sample.The adsorption capacity for sample M94 and M92 shows faster adsorption for initial 30 min and then there was almost negligible change upto 80 min (Fig. 8).Whereas as for other samples i.e MoO 3 , M98 and M96 shows gradual increase in adsorption capacity with respected to time for the given time interval.The adsorption capacity is highest for M92 sample (32.20 mg g −1 ).The adsorption capacity of all samples shows a decreasing order from M92 > M94 > M96 > M98 > MoO 3 .The change in Gibbs free energy calculated using Eq. 3 gives negative values for all the adsorbent samples.This indicates the adsorption is favorable and spontaneous by all the adsorbent samples for the adsorption of MB. 17,29 Removal mechanism of methylene blue.-It was observed that the removal of MB by MoO 3 , M98, M96, M94 and M92 nanoparticles sample was by adsorption mechanism.In general, factors such as steric hindrance from bulk organic molecules, specific surface area, interactions between adsorbent and adsorbate, heterogeneous adsorption sites, and, most crucially, surface charge, influence the adsorption phenomena. 30The negatively charged surface has been used to determine dye absorption by metal oxides. 31,32As the zeta potential of the all samples particles dispersed in DI water was found in range of −36.4 to −44 mV at 0 h and −41.7 to −53.9 mV after     Regeneration and reusability studies.-Regenerationand reuse of the adsorbent material is crucial since it has a direct impact on costs and the material's suitability for continuous batch adsorption operations.In a real-world system, only reusable adsorbent materials are useful.Desorption of MB from the adsorbent was carried out by thermal annealing of the absorbent at 550 °C for 30 min.FTIR spectral study was used to confirm adsorption and desorption of MB from the adsorbent.Figure 10 shows the FTIR spectra of MB, MB adsorbed on M92 sample and regenerated M92 sample after annealing at 550 °C.The signals between 1000 and 1700 cm −1 range corresponds to the typical vibrations of MB bonds. 30,33These signals are only present in MB and MB adsorbed M92 sample FTIR   Figure 11 shows the steps for regenerating the sample from MB solution.Adsorption activity of regenerated M92 sample was carried out using same producer as stated in earlier section of adsorption measurement.Few milligram of fresh M92 sample was added into regenerated Mo92 sample which was lost during the regeneration process.The regenerated M92 sample shows (Fig. 12) the similar efficiency of removing MB from the aqueous MB solution as that of fresh M92 sample.Consequently, the sample can be efficiently renewed and reused again for the adsorption process.

Conclusions
We have successfully identified the effect of V element doping on the MB degradation driven by adsorption process.The negative value of change in Gibb's energy indicates adsorption is favorable and spontaneous for all the adsorbent samples.With increase in V element doping concentration in MoO 3 , the adsorption performance increases and is highest for M92 sample.Such behavior may be due to synergetic effect of surface area and surface charge of M92 sample.The performance of regenerated M92 sample was similar to that of freshly prepared M92 sample which shows the reusability of M92 is possible after thermal treatment.
displays the elemental mapping of Mo, O, and V for the synthesized compounds along with SEM.The EDAX profiles of pure -MoO 3 Fig.2a solely demonstrate the existence of constituent elements Mo and O, ensuring the purity of the MoO 3 sample.While the EDAX profiles of V element doped MoO 3 Figs.2b-2e reveal the presence of V element in addition to Mo and O and gives the evidence of V element doping in MoO 3 .Raman analysis.-TheRaman spectra of α-MoO 3 , M98, M96, M94, and M92 samples are shown in Fig. 3.The peaks observed at 995 cm −1 ((Mo=O stretch) due to Mo 6+ =O asymmetric stretching modes of terminal (unshared) oxygen), 819 cm −1 (doubly coordinated oxygen Mo 2 -O stretching modes of corner shared atoms common to two MoO 6 octahedrons (M=O stretch), 666 cm −1 (Mo 3 -O stretching vibration of triply coordinated bridging oxygen caused by edge shared oxygen atoms (O-M-O stretch), 378 cm −1 (due to O-Mo-O scissoring mode), 337 cm −1 (due to O-Mo-O bending mode), 291 cm −1 (due to wagging mode of O=Mo=O bonds), 285 cm −1 (due to wagging mode of O=Mo=O bonds), 245 cm −1 (O=Mo=O twisting mode) are matches well with the earlier reported Raman data.

Figure 6 .
Figure 6.(a) Degradation of MB with respect to time.(b) Degradation of MB by MoO 3 with respect to time.(c) Degradation of MB by M98 sample with respect to time.(d)Degradation of MB by M96 sample with respect to time.(e) Degradation of MB by M94 sample with respect to time.f) Degradation of MB by M92 sample with respect to time.

Figure 9 .
Figure 9. Schematic of adsorption of methylene blue on sample nanoparticles.

Figure 10 .
Figure 10.FTIR spectra of MB, MB adsorbed on M92 sample and regenerated M92 sample after annealing at 550 °C.

Figure 11 .
Figure 11.Steps to regenerate the sample for aqueous MB solution.

Table I .
Stability behavior and zeta potential dependence of a colloidal solution.

Table II .
Comparison of Zeta potential all the samples.samplesarenegatively charged at natural pH.As a result, significant electrostatic interactions between the negatively charged samples surface and the positively charged MB cations govern adsorption.Figure9depicts a schematic of methylene blue adsorption on sample nanoparticles together with images of color changes

Table III .
Absorption capacity, Removal % and Change in Gibbs free energy.It's evident that surface area plays a vital role in the performance of the adsorption studies.As more the surface area, more are active sites for adsorption process.As the M92 sample has maximum surface area, it may be the possible reason of efficient adsorption by M92 sample as compared to other samples.