Vanadium Pentoxide Nanoparticles Doped ZnO: Physicochemical, Optical, Dielectric, and Photocatalytic Properties

In this work


Introduction
Zinc oxide (ZnO), being a broad bandgap semiconductor, is a potential host for solid-state blue to UV optoelectronics, including laser development [1].Tis has crucial uses in high-density data storage systems, solid-state lighting, in which white light is generated by phosphors triggered by blue or UV light-emitting diodes, secure communications, and bio-detection [2].Transparency to visible light opens the door to the development of transparent electronics, UV optoelectronics, and integrated sensors from the same material systems.Recent advances in the quality and control of conductivity in ZnO have heightened interest in its usage for surface acoustic wave devices, gas sensors, solar cell displays, and window materials [3][4][5][6][7].
Photoluminescence photoconductivity, photocatalytic activities, and absorption studies of ZnO reveal the inherent direct bandgap, a strongly bound exciton state, and gap states attributable to point defects [8,9].Te near-band-edge UV photoluminescence peak at 3.2 eV at room temperature is ascribed to an exciton state, as the exciton binding energy is around 60 meV [10].Furthermore, visible emission is detected due to the presence of defect states.A blue-green emission has been explained in terms of transitions involving self-activated centers formed by a doubly ionized zinc vacancy and an ionized interstitial Zn + [10], oxygen vacancies [11], and donor-acceptor pair recombination involving an impurity acceptor [12].
A key parameter for evaluating piezoelectric performance is the d33 coefcient [13], for pure bulk ZnO, it is 9.9 pm•V −1 .Nevertheless, an advanced piezoelectric device still requires a stronger piezo response.Doping is one of the most efective methods of improving piezoelectric properties.Te use of transition metals (TMs) in ZnO nanoparticles (NPs) shows tremendous potential as a photocatalyst, sensor, optoelectronic, medical, spintronic, and piezoelectric devices [14,15].Indeed, the electronic structure of the host lattice is infuenced by the strong hybridization of 3d orbitals of the transition metal ions with the s or p orbitals of the neighboring anions.Tis hybridization gives rise to the strong magnetic interaction between the localized 3d spins and the carriers in the host valence band.As a result, a giant piezo response of 3d-TM doped ZnO has been demonstrated in several reports; 120 pm•V −1 for Cr-doped ZnO [16], 127 pm•V −1 for Fe-doped ZnO [17], and 86 pm•V −1 for Mn-doped ZnO [18].Among all TMs, ZnO doped with vanadium (V) produces the highest piezoelectric coefcient, 170 pm•V −1 , with a vanadium doping concentration of 2.5 at% [19].Tis value is comparable to perovskites, suggesting that current doped ZnO devices could be improved qualitatively and could be used in a broader variety of applications.Terefore, V-doping is more efective than other TM dopants at improving piezoelectric properties.
A number of studies have revealed that V-doped ZnO exhibits very interesting optical properties, including photoluminescence (PL) and excitation of photoluminescence (EPL) [20,21].Terefore, the application of this semiconductor to visible photocatalysis can be of interest.Many studies have shown that using vanadium pentaoxide (V 2 O 5 ) as a doping element improves the host's structural and optical properties.Nevertheless, the use of doping V 2 O 5 element, as a photocatalyst, is very rarely studied as it is common for vanadium to be used as junction catalysts with other oxides; supported-vanadium-oxide-catalysts, such as Al 2 O 3 , SiO 2 , TiO 2 , and ZrO 2 [22].
Herein, the efect of vanadium-(V-) doped ZnO on dielectric properties besides the photocatalytic performance of the prepared materials was studied.Te prepared materials were characterized by X-ray difraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy combined with energy-dispersive spectroscopy (SEM-EDX), and UV-visible spectroscopy.Te dielectric properties of samples were studied using complex impedance spectroscopy.

Synthesis of Vanadium-Doped ZnO.
Vanadium-(V-) doped zinc oxide samples were prepared using a solid-state reaction.Synthetic V 2 O 5 nano-powder was added to commercial nano-powder ZnO in various molar percent, then combined and mashed for 30 minutes in a high-speed ball milling (Ritsch model) equipped with MTI mufe furnaces operating at 1000 rpm.After that, the samples were compacted into pellets, placed in alumina crucibles, and heated in a furnace at 500 °C for 6 hours.After cooling to room temperature, they were ground manually for 30 minutes.Te obtained samples bearing the acronym 3 V, 5 V, 7 V, and 9 V for 3, 5, 7, and 9 molar percent of V 2 O 5 , respectively.

Characterizations.
Te obtained samples were structurally characterized using XRD analysis.Te difracted data were collected on an automatic difractometer Philips Panalytical X'Pert ProMPD using a Cu Kα radiation source (λ �1.5406 Å) in the 2θ range of 10-80 °.Fourier-transform infrared spectroscopy analysis for the samples was carried out on an IRAfnity-1S from Shimadzu.Te samples were vacuum dried and then analyzed with FTIR spectroscopy in the range of 4000 − 500 cm −1 wave numbers.Scanning Electron Microscopy coupled with energy-dispersive X-ray was performed using JEOL 2010 (200 kV) microscope.UV-visible difuse refectance spectroscopy (UV-vis DRS) measurements were performed using an AvaSpec-2048 Fiber Optic Spectrometer with a symmetrical Czerny-Turner design.Spectra were recorded in the range of 200-1000 nm using a 2048-pixel CCD detector array.Te dielectric properties of prepared materials were studied using complex impedance spectroscopy (CIS) Palmsens4 from Palmsens.Te samples are pelletized with a 13 mm diameter and a 2 mm thickness and then placed between two glasses coated with conductive FTO electrodes.Te complex impedance measurements were carried out at room temperature by scanning the frequency from 1 Hz to 1 MHz with a voltage of 5 m V RMS.

Photocatalytic Performance under Visible Light.
Te photocatalytic performance of the prepared catalysts was evaluated by employing the degradation of methylene blue (MB) under visible light.Typically, 30 mg of the catalyst was loaded into a beaker containing 30 mL of MB solution (20 mg/L).Te experiments were carried out in the dark for 40 min to achieve the adsorption-desorption equilibrium.Te blank experiment (without catalyst) was also conducted under visible light to evaluate any possible direct photolysis of the dye.Photodegradation was performed using a HQI-E 400 W/n plus visible lamp and maintained under magnetic stirring >1200 rpm to overcome any external difusion.At diferent time intervals, samples were drawn from the reaction medium and analyzed using a Shimadzu UV-1900 UV-vis spectrophotometer over the 200-800 nm range.Te degradation efciency was calculated using the following equation: where C 0 and C t represent the concentration of MB at the initial time (t � 0) and reaction time (t ≠ 0), respectively.1.

Results and Discussion
Based on the data presented in Table 1, a systematic increase of both unit cell parameters a and c with increasing the molar percent of vanadium (V) oxide.Tese results can be explained by comparing the ionic radii for tetrahedral coordination of Zn 2+ (0.88 Å) and V 2+ (0.93 Å).Hence, the substitution of zinc for vanadium in the ZnO matrix induces an expansion of unit cell volume.
Te crystallite sizes "D" was calculated through either Scherrer or Williamson-Hall equations as follows: Scherrer equation: where k � 0.89 is a constant, λ � 1.5406 Å, θ, and β are the difraction angle and the corresponding full width at halfmaximum FWHM of the observed peak, respectively.Williamson-Hall equation: where ε is the strain of the samples.Figure 3 depicts the variance of β cos θ against 4 sin θ for all samples except sample 9 V. Te full width at a halfmaximum of 9 V cannot be accurately determined due to the X-ray fuorescence which is more pronounced with a high molar percent of vanadium oxide.For all samples, a small variation of strain is observed (Table 2).It may be attributed to lattice distortion due to the substitution of Zn 2+ (0.88 Å) by vanadium V 2+ (0.93 Å) in the zinc oxide matrix.Santi Maensiri [23] reported comparable results.Te crystallite size calculated in equations ( 2) and (3) presents the same trend.It actually decreases with increasing vanadium molar percent.Te discrepancy in D values between the Scherrer and Williamson-Hall methods can be explained by the fact that the Scherrer methodology ignores strain.

UV-Vis Difuse
Refectance Spectroscopy.Te UV-vis difuse refectance spectroscopy was employed in this work to investigate the energy structures and optical characteristics of V 2 O 5 -doped ZnO samples.Figure 4 illustrates the difuse refectance spectrum of prepared samples.Tey display a dramatic rise in refectance at 370 nm.
Te band gap energy values were evaluated using the Kubelka-Munk method.Te bandgap energy (E g ) is a critical property of semiconductor materials.Te value of E g , like other physical and chemical characteristics of materials, their value may be modifed by numerous aspects such as the synthesis pathway and/or processing method.In this regard, optical characterization techniques combined with the well-known Tauc method are by far the most extensively used approach for determining the E g of supported materials [24].It has been demonstrated that when properly conducted, this approach yields precise values of E g [25].Te Tauc charts are created using DRS by computing the reemission function, where R ∞ is the refectance of the sample with "infnite thickness," hence, there is no contribution of the supporting material, K, and S are the absorption and scattering K-M coefcients, respectively, C is a proportionality constant.α represents the absorption coefcient of the material, h denotes Planck's constant, ] refects the frequency of light, h] is the energy of the incident photon, and n is a coefcient that depends on the kind of electronic transition; n � 1/2 for direct allowed transition, n � 3/2 for direct forbidden transition, n � 2 for indirect allowed transition, and n � 3 for indirect forbidden transition.In our situation n � 1/2: for direct transition, mode materials), because ZnO is a direct bandgap semiconductor [26].Te Tauc approach basically fnds the value of E g at the intersection of the straight-line ft of the region associated with the optical absorption edge (i.e., the Tauc segment; TS) and the h] -axis in the related (αh]) 2 vs. h] plot (Tauc plot) [27][28][29].
Trough equations ( 4) and (5) the bandgap (E g ) is calculated by constructing a Tauc plot (Figure 5) from the refectance spectra.
Table 3 lists the E g values for each sample.It is found that the bandgap of V 2 O 5 /ZnO has little change consistently with the increasing concentration of V 2 O 5 .Tis occurs because V 2+ ions take the place of Zn 2+ ions in the lattice crystal, implying that V 2+ ions contribute to the valence band of ZnO materials.Tese results are comparable to those encountered by previous researchers [30].Tis scenario decreased Zn-O bonding while promoting V-O development.
International Journal of Biomaterials Furthermore, variations in bandgap values for V 2 O 5 /ZnO may be additionally attributed to grain size, strain, structural parameters, and composition [31,32].Te quantum confnement efect caused by the tiny size is responsible for the blue shift in the optical bandgap.

FTIR Spectroscopy.
Te infrared spectra of the prepared samples are depicted in Figure 6.Symmetrical vibration of Zn-O appears to blow 500 cm −1 [33] (not seen here).Te absorption bands detected in the other sample spectra between 750 and 950 cm −1 are attributable to the symmetrical and asymmetrical stretching vibrations of V � O bonds.Te absorption band observed around 650 cm −1 corresponds to V-O-V bending vibration.Te corresponding intensities increase systematically with the molar percent of V 2 O 5 .

SEM-EDX Analysis.
Figure 7 shows SEM images of the samples.V 2 O 5 nano-powder morphology presented as an aggregated parallelepiped with a smooth surface.Individual particles of various shapes in the instance of ZnO nanopowder are observed.Large spheres, as well as tiny particles, were seen in 3 V, 5 V, 7 V, and 9 V. Particle aggregation occurs in all samples, which may be caused by hightemperature calcination.
Te EDX peaks demonstrate exclusively the elements of the samples, i.e., oxygen, vanadium, and/or zinc.For clarity, only 7 V is presented in Figure 8.As expected, the weight percent of vanadium in the samples determined by EDX is in good agreement with starting composition (Figure 8 inset table).

Photocatalytic Degradation of Methylene Blue (MB) Dye.
Te photocatalytic performance of the as-synthesized photocatalysts 3 V, 5 V, 7 V, and 9 V, as well as the pure ZnO, was conducted by testing MB photodegradation in an aqueous solution under visible light irradiation.In the initial stage, a blank test was performed using MB solution, with no       photocatalyst under visible light for 420 min.As shown in Figure 9(a), the UV-vis spectra show no change in the peak intensity of the dye (at 665 nm), suggesting that the MB cannot be efectively degraded by photolysis.In the presence of the photocatalysts, the absorption of MB dye gradually reduced with increasing irradiation time (Figure 9(b)).Te photodegradation percentages of the catalysts were 70% (0 V), 37% (3 V), 82% (5 V), 90% (7 V), and 78% (9 V), after 420 min irradiation.Figure 9(c) shows the variation of C t /C 0 of MB as a function of time under visible light irradiation in the presence of 0 V, 3 V, 5 V, 7 V, and 9 V photocatalysts.Interestingly, in the presence of the catalyst under dark conditions, the experiments indicate that after 40 min the adsorption of the dye was found to be 27% for 3 V while the other photocatalysts exhibit negligible adsorption toward dye (less than 2%).Tis behavior may be credited to the aggregation of V 2 O 5 -ZnO and the subsequently reduced surface area of the catalyst [34].
Te photocatalytic degradation under visible light indicates that the ZnO doping with V 2 O 5 enhances its photocatalytic performance.Te best result was obtained in the presence of a 7V catalyst.Tese results could be attributed to the bandgap narrowing as seen from UV-vis DRS spectra.Moreover, from the XRD the particle size was found to be decreased with the increase in the V 2 O 5 concentration, thus as the particle size decreases, the specifc surface area increases then the photocatalytic performance increase [35,36].Generally, the improvement in photocatalytic activity upon V 2 O 5 doping can be ascribed to the fact that the dopant species absorbs the generated electrons through irradiation and then transfers them to the oxygen molecules [34].Doping ZnO with V 2 O 5 dopant allows the separation of electron-hole pairs, thus enhancing the photocatalytic performance.Any further increase in the V 2 O 5 concentration led to a decrease in the photocatalytic degradation efciency.In link with XRD results, sample 9 V clearly shows 6 International Journal of Biomaterials the presence of Zn 3 (VO 4 ) 2 as an impurity that may act as a recombination center [37].Te degradation of the MB solution under visible light irradiation was examined to ft on pseudo-frst-order reaction kinetics as follows: C 0 presents the initial concentration of the MB and C t is the MB concentration at time t ≠ 0. k 1 is the pseudo-frst-order rate constant.Te photodegradation rate constant k 1 (min −1 ) was calculated from the slope of the straight line of ln(C t /C 0 ) versus time t.Te degradation of MB over the prepared materials is best interpreted by the pseudo-frst-order kinetics model (Figure 9(d)).Te kinetics parameters are gathered in Table 4.
By increasing the V 2 O 5 content, the initial rate increases reaching the maximum value (0.010 mmol•min −1 •mg −1 ) for the 7 V catalyst then decreased when reached 9% of V 2 O 5 (Figure 10).Generally, the reactive species generated during irradiation of photocatalysts are holes (h + ), hydroxyl radicals (OH • ), and superoxide radicals (O 2 •− ) which mainly govern the photocatalytic mechanisms.
In the frst step, a V 2 O 5 /ZnO photocatalyst absorbs visible light energy at an appropriate wavelength to create electronhole pairs.During the photo-generation process, electrons are   •− to form, and then, OH • to be formed upon their conversion into energy.Conversely, holes in the valence bond of ZnO interact with the adsorbed H 2 O or hydroxyl groups to produce surface hydroxyl radicals [36].
For a better understanding of the mechanism of MB photocatalytic degradation over prepared materials, the type of reactive species that play the most signifcant role in photocatalytic degradation must be identifed.For this purpose, these reactive species were investigated by adding (4 μmole) of diferent organic scavengers to the reaction medium in the presence of the catalyst.For holes (h + ) trapping, we use ethylenediaminetetraacetic acid (EDTA), isopropanol alcohol (IPA) for hydroxyl radicals (OH • ), and L-ascorbic acid (ASC) for superoxide radicals (O 2 •− ) [38,39].Te catalyst 7 V was used as a model catalyst in these experiments.
Te efect of adding diferent scavengers on the photocatalytic efciency is illustrated Figure 11.As seen, the photocatalytic efciency of the MB degradation over 7 V was reduced from 90% (without adding a scavenger) to 62% after adding ascorbic acid indicating that the (O 2 •− ) active species play a relatively minor role the photocatalytic degradation process compared to other reactive species.On the other hand, it can be observed that after the addition of isopropanol, the photodegradation performance of MB was negligible (about 2%), while with the addition of EDTA the MB degradation was reduced by 29% compared to the catalytic performance without adding a scavenger.Terefore, the photogenerated holes in the valence band of the catalyst are trapped by H 2 O molecules to give hydroxyl radicals.Hence, the photogenerated holes and hydroxyl radicals could be the most efective active species with an essential role in the photocatalytic process.Moreover, the presence of orthorhombic phase Zn 3 (VO 4 ) 2 as secondary phase may act as a recombination center hole electron pair [37].As a result, a drop of photocatalytic activity is observed for the sample 9 V containing the highest weight percentage of Zn 3 (VO 4 ) 2 .In addition, from Figure 9(b), we can observe that there is a blue shift of the main peak at λ � 665 nm which is shifted from 665 nm to 661 nm during the reaction.Generally, these types of blue shifts can be attributed to the diferent N-demethylation steps for MB degradation [40], this led us to suggest that the MB is degraded into fnal products in multiple steps.
3.6.Dielectric Spectroscopy.Te complex impedance spectroscopic (CIS) approach is used to assess the electrical response of materials across a broad frequency and temperature range.It is the most common used approach for studying the dielectric behavior and dynamics of ionic transport in materials.In fact, this methodology is particularly benefcial in evaluating the contribution of various processes in the conduction process.As a result, the physical process occurring within the sample may be represented as an analogous circuit using impedance spectra.In Figure 12, the electrical response of the material was represented using a Nyquist diagram (spectra of the real part and the imaginary part of the complex impedance, Z * (Z * � Z ′ + jZ ″ ; j 2 � −1), as a function of the exciting frequency).Scattered experimental points are arranged in an approximately circular arc.Tis response can be associated with an electric dipole formed by resistance in parallel with a capacitor.However, since the scattered points do not form exactly half circles, a model combing only resistance with a capacitor element is insufcient to describe accurately the CIS results.Hence, a Constant Phase Element (CPE) in parallel with a resistance R is usually regarded as a better representation of the circuit-ftting parameters [41].Te equivalent electrical circuit of the samples is shown in the inset of Figure 12.Te red line represents the best ft.
where the impedance of the CPE is defned as where ω is the angular frequency (ω � 2πf), A 0 is a constant independent of frequency [42], and 0 < n < 1 is a dimensionless parameter determining the degree of deviation from an exact semicircle [43].When n � 1, equation ( 8) yields the impedance of a capacitor, where A 0 � C. Te resistance R is the intercept of the impedance curve with Z ′ axis.Te experiments have been ftted by the ORIGINLAB software based on the following relationships.
1 + 2RA 0 ω n cos(nπ/2) + RA 0 ω n  2 . ( Figure 12 shows that increasing V 2 O 5 loading induces a decrease in the capacitance C as well as the resistance R. Te extracted parameters from the ft for the circuit elements are collected in Table 5. For each loop, a time constant τ (lifetime at the depletion layer of the semiconductor) is equal to the product of the resistance and the capacitance associated with the defned loop (τ � RC).Going from 0 V to 9 V leads to a decrease in the resistance from 1547 MΩ to 321 MΩ as well as the capacitance from 14.31 pF to 2.75 pF.Accordingly, the electron lifetime decreases from 22.13 ms to 0.88 ms.

Conclusion
Vanadium oxide-doped zinc oxide samples were prepared via a solid-state reaction process.Te efect of V 2 O 5 on the ZnO physical properties and photocatalytic performance was reported.Te XRD shows that by increasing the concentration of dopant (V 2 O 5 ) a minor orthorhombic phase assigned to Zn 3 (VO 4 ) 2 appeared at 9% of V 2 O 5 and may act as a recombination center in the photocatalytic degradation of methylene blue dye.Dielectric spectroscopy analysis reveals that by increasing V 2 O 5 , loading the capacitance (C) decreases as well as the resistance R. Te photocatalytic degradation under visible light indicates that the ZnO doping with V 2 O 5 enhances its photocatalytic performance, where the best result of 90% MB degradation was obtained in the presence of 7 V catalyst.

Figure 1 :
Figure 1: X-ray difraction (XRD) pattern of the samples.Te pattern of pure ZnO is given for comparison.

Figure 2 :
Figure 2: Rietveld refnement of the XRD pattern of 7 V sample.

Figure 9 :
Figure 9: Photocatalytic degradation of MB over prepared materials under visible light, (a) without catalyst, (b) UV-vis spectra of MB degradation over 7 V catalyst, (c) change in the concentration of MB versus time, (d) pseudo-frst-order kinetics plot of the photodegradation of MB in the presence of prepared catalysts.

1 Figure 10 :Figure 11 :
Figure 10: Change in initial rate versus the content of V 2 O 5 .
Figure 1 depicts the powder XRD pattern of the samples collected at room temperature.Te XRD pattern reveals that a solid-state reaction has occurred by displaying a secondary phase.Compared to pure ZnO, all doped samples have a dominated hexagonal wurtzite phase and a minor orthorhombic phase assigned to Zn 3 (VO 4 ) 2 according to the JCPDS340378 card clearly shown in sample 9 V. Te hexagonal wurtzite of V 2 O 5 doped ZnO solid solution presents unit cell parameters a � b � 3.253 Å and c � 5.213 Å (JCPDS01089139).Te orthorhombic crystalline structure of Zn 3 (VO 4 ) 2 is characterized by a � 8.299 Å, b � 11.52 Å, and c � 6.111 Å. Te unit cell parameter of all samples was refned via the Rietveld refnement method, (full pattern matching option) using the FULLPROF program.For clarity reasons, only 7 V sample results are presented in Figure 2. Te ratability factors are χ 2 � 1.43, R p .� 14.8, R wp .� 21.1, and R exp .� 17.64.Tese values indicate a good agreement between the calculated and experimental data.Te unit cell parameters for all samples are summarized in Table 3.1.X-Ray Difraction.

Table 1 :
Unit cell parameters and volume determined by Rietveld refnement.

Table 2 :
Crystallite size and microstrain calculated using the Debye Scherrer and Williamson-Hall methods.

Table 3 :
Bandgap (E g ) of the prepared materials.

Table 4 :
Pseudo-frst-order kinetic parameters of MB degradation over prepared materials.O 5 , whose potential is below ZnO's conduction band.As the electrons trapped on the surface of the V 2 O 5 react with the dissolved oxygen, they react with the reactive oxygen species, converting H 2 O to OH • .Meanwhile, V 2 O 5 enhances charge separation for photogenerated charges.As the photogenerated electrons on the ZnO surface are trapped by dissolved O 2 , they cause O 2